During the game I went to this season I was able to sit in the MUSS no problem, even without a MUSS ticket. The closer to the field you are the more protective people will be of their seats, so if they don’t mind being a bit further up there should be no issue. I have no idea if they’re actually checking IDs for MUSS tickets, but my feeling is they should be fine.
Besides, if the tickets are free what is the worst that can happen? The MUSS is a great time if you’re with friends–I say go for it!