The UW can tax Ceres real gold, not fiat paper continentals
As for taxing them gold. Gold is worthless, why would they want gold? Outside of a handful of industrial or tech uses there is no value to gold.
Tell that to people who search for pirate gold.
When you have gold it means you're a winner, not a loser. If you can pay a blackmailer or a kidnapper with gold it means you're worth blackmailing or kidnapping.
Diamonds would be even better -- they can be far more valuable per gram -- but most people need a diamond appraiser they trust before they even know what they have. Without an expert you can't tell whether a diamond is flawed or synthetic. So they aren't as good after all.
If people are poor and they think the UW is bankrupt they're likely to revolt. If they are poor and they think the UW has lots of gold they won't. Does it make sense? No. People often don't make sense. But often they can predict what other people will do whether it makes sense or not. The UW can survive if it has a pile of gold to sit on like a dragon, and it's likely to fail otherwise.
What if they could somehow pretend they got a big pile of gold from the Belt, and nobody particularly denies it? Would that be enough? Yes. It probably would.
I can imagine you objecting to this. "It doesn't make sense! Why would people put up with a greedy UW that doesn't share its (fake) gold with them, when they wouldn't put up with a feckless UW that spends more than it can tax from them?" I'm sorry. It doesn't make sense.
If people made sense they'd probably have a lot less government.