That would only seem workable if the donors could offset the amount of donation from their tax liabilities. Also, most BI discussions I've seen are about how to replace the existing benefits delivery infrastructure with BI, and not as an additional entitlement.
Blisterpeanutes: "People who support blind charity should put their money where their mouth if they think it's such a great idea."
Yourapostasy: "Hey great idea, even better why don't we give them all the money they give back so they don't actually have to make a sacrifice at all for their beliefs!"
If you want a bifurcated benefits distribution program, where BI and "conventional" taxes exist side-by-side, to see which program is more effective in an open competition, then telling supporters of BI "you not only have to pay the taxes you already pay, but you also then pay beyond that for BI" would handicap the results in favor of the existing "conventional" taxation system. I personally think advocates of BI are committing the Soros reflexivity mistake, which pretty much most of the field of economics sins against as well. Without a near-free energy source, I doubt BI works over the long-haul. But that doesn't mean when someone advocates testing BI by telling supporters to dig into their pockets even deeper after they have paid taxes they currently are liable for, that I will idly stand by and let that pass for a fair challenge. Operationally, unless there is some strong way (like via DNA) to identify benefits recipients of one system over another to avoid double-dipping and similar negative externality behavior, I don't think side-by-side systems are actually feasible.
