Hi! these suggestions are based on part of our necessities (we're an NGO with special projects based on agriculcultural initiatives). Explanations:
1 - To Fix an amount for a participation. For example, into the project X, 1 participation = 20EUR
2 - The button could have an associated input field (or progressive bar) in order to indicate the quantity of participations the user want to "donate". For example, 2 participations (2*20=40EUR).
3 - The payment could be recurrent or not (admin setting for a campaign)
This point of view is terribly positive to new approaches into organic agricultural projects. We can base this participations, for example, to dealing agro-goods for our members, in the term of "you support this project to Conservation of Native Breeds, and will receive 6 eggs a week by month"
3 - Another need are related with member profiles. Will be good to have "connection" between the supported projects by members and participations. For example: I created N total projects. X=N-Y projects are configured to appear on member profile (checkbox list + participation tags or input fields-> read-only fields and checks). The member can see about he is supporting 3 of X projects, with 1, 4, and 2 participations into each one. For another part, admin can query about supporting-state of projects.
I installed the free version of your plugin. It fits our basic needs. With these suggestions, it could be the "super-plugin" for our purposes.