Ive done so on the wild grapes, no noticeable issues. Would guess it is OK with wine grapes since Mosti is selling frozen must through George,cant imagine a huge difference in the juice or the grape being frozen, maybe someone else knows more.
I freeze my homegrown grapes. The book I use recommends freezing all the fruit that you use tomake wine. I don't know if this is right but it has worked for me.