Be as open about your organization's goals as you can without compromising business secrets. If you want the project to acquire a certain feature because, say, your customers have been clamoring for it, just say so outright on the mailing lists. If the customers wish to remain anonymous, as is sometimes the case, then at least ask them if they can be used as unnamed examples. The more the public development community knows about why you want what you want, the more comfortable they'll be with whatever you're proposing.
This runs counter to the instinct — so easy to acquire, so hard to shake off — that knowledge is power, and that the more others know about your goals, the more control they have over you. But that instinct would be wrong here. By publicly advocating the feature (or bugfix, or whatever it is), you have already laid your cards on the table. The only question now is whether you will succeed in guiding the community to share your goal. If you merely state that you want it, but can't provide concrete examples of why, your argument is weak, and people will start to suspect a hidden agenda. But if you give just a few real-world scenarios showing why the proposed feature is important, that can have a dramatic effect on the debate.
To see why this is so, consider the alternative. Too frequently, debates about new features or new directions are long and tiresome. The arguments people advance often reduce to "I personally want X," or the ever-popular "In my years of experience as a software designer, X is extremely important to users" or "...is a useless frill that will please no one." Predictably, the absence of real-world usage data neither shortens nor tempers such debates, but instead allows them to drift farther and farther from any mooring in actual user experience. Without some countervailing force, the end result is likely to be determined by whoever was the most articulate, or the most persistent, or the most senior.
As an organization with plentiful customer data available, you have the opportunity to provide just such a countervailing force. You can be a conduit for information that might otherwise have no means of reaching the development community. The fact that the information supports your desires is nothing to be embarrassed about. Most developers don't individually have very broad experience with how the software they write is used. Each developer uses the software in her own idiosyncratic way; as far as other usage patterns go, she's relying on intuition and guesswork, and deep down, she knows this. By providing credible data about a significant number of users, you are giving the public development community something akin to oxygen. As long as you present it right, they will welcome it enthusiastically, and it will propel things in the direction you want to go.
The key, of course, is presenting it right. It will never do to insist that simply because you deal with a large number of users, and because they need (or think they need) a given feature, therefore your solution ought to be implemented. Instead, you should focus your initial posts on the problem, rather than on one particular solution. Describe in great detail the experiences your customers are encountering, offer as much analysis as you have available, and as many reasonable solutions as you can think of. When people start speculating about the effectiveness of various solutions, you can continue to draw on your data to support or refute what they say. You may have one particular solution in mind all along, but don't single it out for special consideration at first. This is not deception, it is simply standard "honest broker" behavior. After all, your true goal is to solve the problem; a solution is merely a means to that end. If the solution you prefer really is superior, other developers will recognize that on their own eventually — and then they will get behind it of their own free will, which is much better than you browbeating them into implementing it. (There is also the possibility that they will think of a better solution.)
This is not to say that you can't ever come out in favor of a specific solution. But you must have the patience to see the analysis you've already done internally repeated on the public development lists. Don't post saying "Yes, we've been over all that here, but it doesn't work for reasons A, B, and C. When you get right down to it, the only way to solve this is Q." The problem is not so much that it sounds arrogant as that it gives the impression that you have already devoted some unknown (but, people will presume, large) amount of analytical resources to the problem, behind closed doors. It makes it seem as though efforts have been going on, and perhaps decisions made, that the public is not privy to — and that is a recipe for resentment.
Naturally, you know how much effort you've devoted to the problem internally, and that knowledge is, in a way, a disadvantage. It puts your developers in a slightly different mental space than everyone else on the mailing lists, reducing their ability to see things from the point of view of those who haven't yet thought about the problem as much. The earlier you can get everyone else thinking about things in the same terms as you do, the smaller this distancing effect will be. This logic applies not only to individual technical situations, but to the broader mandate of making your goals as clear as you can. The unknown is always more destabilizing than the known. If people understand why you want what you want, they'll feel comfortable talking to you even when they disagree. If they can't figure out what makes you tick, they'll assume the worst, at least some of the time.
You won't be able to publicize everything, of course, and people won't expect you to. All organizations have secrets; perhaps for-profits have more of them, but nonprofits have them too. If you must advocate a certain course, but can't reveal anything about why, then simply offer the best arguments you can under that handicap, and accept the fact that you may not have as much influence as you want in the discussion. This is one of the compromises you make in order to have a development community not on your payroll.