Big donors, yes. But firms with this motivation are already involved. I think the problem is how to engage the long tail of small / medium firms that rely on OSS but aren't sufficiently engaged to try to shape them.
Actually, you could get the government involved here. There could be a tax break for the first $X of contributions to open source. That would effectively make a public subsidy to OSS, directed by market forces. Pretty cool
FLOSS currently gets extra scrutiny ("BOLO", which usually means "no") when applying for 501c3 status. The reasoning is to avoid a situation like the following:
Google has some extremely-Google-specific work they want done. Google spins up a project to do it as an external entity, and "donates" the money to make it happen. Google takes that as a tax deduction, and meanwhile the 501c3 gets all the other tax benefits a 501c3 gets (often property tax exemption, for instance).
This is a fair objection, to a point - an organization whose sole purpose is to benefit a for-profit company should probably be paying property tax. I think they take it too far - "Be On Look Out" is appropriate, but it shouldn't mean "no" when projects are genuinely creating value that is broadly accessible.
Your narrower program described above also solves the problem, albeit arguably in a patchier way. The money from Google that would be spent on business expenses anyway wouldn't have been taxed anyway.
Actually, you could get the government involved here. There could be a tax break for the first $X of contributions to open source. That would effectively make a public subsidy to OSS, directed by market forces. Pretty cool