voluntary regressive tax

How is it regressive? (I suppose if A has a policy of giving 10% of what you get after tax and B looks at A's giving as a fraction of income before tax then it'll look like a regressive policy. But you could equally say that if A gives 10% of income before tax, and B looks at giving as a fraction of income after tax, then proportional giving looks like a progressive policy.)

Many advocates of proportional giving would say that if you're poor then you shouldn't be feeling obliged to give at all, which would make the policy progressive overall.

but capital gains are exempt

I don't think it's fair to blame this on the idea of proportional giving.

If we didn't have a culturally accepted obligation for charity, we [...] might be more willing to consent to a higher progressive tax.

I suppose that's possible in theory. I gravely doubt it would actually happen in practice. (Perhaps if we were forbidden to give to charities privately, but that seems like an obviously really terrible idea.)