If authorities wanted to freeze a subset of DAI, could they do so by directing USDC to refuse to back those tokens?

There aren't USDC backing the DAI for any given DAI. If I have 5 DAI, you can't point to 3-4 USDC backing it (I think ETH backs it too).

Even if Circle did freeze some of DAI's USDC, the Dai contract does not contain the ability to freeze user funds, and I believe is also not upgradeable

Nope they'd have to freeze the entire collateral that Dai has (~33% of all collateral they have rn), Dai could not freeze individual addresses and almost assuredly wouldn't even if asked.

Ok, so, "blacklist these particular addresses or we will seize your entire collateral"?