Nothing in our Submissions Agreement prohibits this, so you're good.
With regard to the general question, here's my personal two cents:
I think it's less effective BUT a bit more honest/direct to have the "Download" link be separate from the "Donate/Support" link, to make it explicitly clear that the music in question is being offered for free. My understanding of Bandcamp's policies is that you have X number of credits/points (or something like this?) before you can no longer offer music for free... this bothers me, as does using Bandcamp as a platform for free releases, because they're ultimately a for-profit company, they're motivated by $$$ (not a bad thing, just saying), and they don't seem like a great PERMANENT home for free releases because of these reasons. Nevertheless, they're easy, and they're popular...I just hate to think of a future years from now where people single-sourced their free stuff on Bandcamp and because of changed policies or expired accounts or running out of credits, it's no longer available.