Essentially, developers/project have to opt-out from participating on your site.
This is the problem that these "fund open source" projects run in to time and time again. Gittip (if I remember correctly) had the same issue. You're accepting funds under the name of various open source projects but not necessarily funding them. Often developers aren't even aware of the parallel funding channel and are rightfully upset about it.
I'm not saying your intentions are bad, and as an open source developer I'd love for a funding model that worked. I've just seen tens of different iterations on this same idea and none have stuck.
This is the problem that these "fund open source" projects run in to time and time again. Gittip (if I remember correctly) had the same issue. You're accepting funds under the name of various open source projects but not necessarily funding them. Often developers aren't even aware of the parallel funding channel and are rightfully upset about it.
I'm not saying your intentions are bad, and as an open source developer I'd love for a funding model that worked. I've just seen tens of different iterations on this same idea and none have stuck.