Hi,
HD wallets like Trezor create addresses that are compatible with certain cryptos, such as ether, litecoin, and others. For ether, there is no reason to create an ether wallet since Trezor integrates with the MEW website.
My question is, instead of creating an online Ether wallet (or other supported altcoin), and then importing it to Bisq, would it be possible to use an address generated from a hardware wallet such as Trezor for an Altcoin wallet on Bisq?
I think you might have misunderstood trading altcoins on Bisq. When using BTC as base currency (which is the default) you only have Bitcoin internal wallet in your app and altcoins are traded the same way fiat is (outside the app). So there are no altcoin internal wallets in Bisq.
I believe that you can change your base currency to Litcoin or Dash however and then do trades on that separate network. I don’t think that you can import any keys to Bisq however.
Other then that, when you are doing an altcoin trade Bisq can not know (and doesn’t care) where you generated your address that you specified for your altcoin account.