Make it as organized/structured as possible. Go through the social initiatives arm of the firm, if they are big enough to have one. To them "our social responsibility initiatives" or "open source community contributions" sounds better and webpage-worthy than "we pay project X, Y and Z" (even though that is basically what it boils down to).

On the other hand, for a smaller firm, this might probably be just a one-to-one talk/convincing an executive. If you can't convince one, try another. If you can't convince any, take it as a flag with the color of your choosing.