As far as I know, that’s not possible.
No you can’t donate to the project itself.. BUT you can of course donate to plugin developers, and by doing this you’re keeping the BP development community healthy 
Donating even a small amount to plugin authors (whose plugins you use on your site) is a great way to thank them and encourage further development. I try to do this myself quite often when I use a new plugin on a client site, etc.
I have so many people I want to donate to and want to thank for all the tools & services provided that we would be charged big bucks for! But if you can’t find where to donate to BP – which I must say is really awesome of those who do want to support services by donating, I agree with donating to authors & devs – because no two frameworks are alike, and these tools are what make them possible & unique for all of us!
smuda,
Seems like you could also donate to a developer to add some core functionality that’s on the roadmap but hasn’t been given resources. This would be like donating to BP.
