Wait, so I'm confused what happens when you don't have any bits at all, the site would still function normally (i.e. you could still circledot someone)? If that's the case I think I'd rather just send money directly to Hubski instead.
EDIT: actually, I take that back - This still seems like a good idea if it generally helps to improve the quality of submissions and comments on the site.
Either way, maybe there could be a donation status somewhere like reddit does with its reddit gold - shows a progress bar-like status of how much money they've gotten/desired per day?