I would not even consider asking. I would tell them I'm going to Keepsake Quilting; I will have no time limits or distractions while there; I will stay as long as I like. I'd possibly try to be polite about it but it would be a "no exceptions" part of the trip. Hopefully she will like it, too; or like you enough that she will be happy for you to enjoy it.
I nearly always travel alone, and always do my limited "fun shopping" alone. The last trip we made to the east coast was in separate vehicles because I had shopping/sight seeing plans, and I did not want to risk not being able to escape from the MIL.