As GRIN recently switched to a more simple payment methods, I think Bisq should support GRIN trading. If I remember correctly, it was disabled because sending GRIN was confusing for people.
I think that Bisq should have a special UI for Bisq trades. For example, internet links on how to send and receive GRINS (official GRIN docs) and also allowing users to exchange SLATEPACK addresses and messages. This could be done by instructing them to exchange it in the chat.
As I said, I think that GRIN trading on Bisq is disabled atm. Imo, now it’s the best time to give GRIN another try. It is a very unique currency. That’s why I think that the trading UI should be modified for GRIN, at least a little bit (even if it’s one pop up message telling people how to trade GRIN).