I also think the "sprint" idea is a really good one, while donators vote for "top wanted features" to be coded during the sprint. It could also be helpful to do the following for each feature:

1. Note what needs to be done (what needs to be implemented). This is to give donators an idea of what they're "paying" for (they deserve to know when the feature isn't That simple to code), but also so that potential developers who don't know how to contribute could see the top listed features easily and know where to start.

2. Estimate roughly how much work it'd take (at least try to note if it'd take a long time or just a couple of hours). If you organise a sprint, this means donators will better understand why Feature X hasn't been completed yet (because it takes "1000 man-hours" to finish all the dependencies).

3. Also describe clearly what the feature does and how the user benefits. Inkscape features can be pretty technical (Cairo), but users will understand if you say "You can use it to accomplish [cool effects]/[serious office work]/[compatibility with other software]/etc., or "It will speed up parts of Inkscape by 20%." Or even "doing this will allow us to then implement Feature Y that all of you want."

4. Progress should be documented clearly, like on a blog or such.

5. As an alternative to sprints, some or all developers could agree to all commit to working on the top feature (without meeting up), either periodically or when total donations reach a certain milestone.

I'll also mention as a side note that although bounties never worked for Gimp and Inkscape, paid development did work for Krita last year. What they did was get a developer (Lukáš Tvrdý) to develop for Krita full-time during several months thanks to donations. Krita has a blog, so every week work on Krita development, including Lukáš', was documented (when money is involved, transparency is important). You can read the log on krita.org (start from the last page). Lukáš coded and improved several brushes engines, improved speed, fixed lots of bugs, etc. I think anybody who observed the progress will agree that Huge progress was made thanks to that, it was very exciting to read week after week.

Of course, it's not an arrangement for everyone, it just happened that a capable developer could be made to work full time on Krita thanks to donations, so it can't necessarily be reproduced for Inkscape. It's just an example of donations that can work, in case you decide not to go for a sprint.