ABSOLUTELY, tipping still works! That is how I know about getting close to the stage at Mama Mia....I booked our MM seats at the last second for a LV trip last March. We were up in the second level (still good seats, but not close). So I made Darrell take me over to Mandalay Bay's showroom an hour before showtime, and I quickly made friends with the older usher stationed at one door. We talked about the "old days" of Vegas (remember I grew up in LV) and compared a few of our favorite off-strip restaurants. When he was allowed to open the doors, he took us up to our seats and quietly said "I'll try to come back". As soon as the lights had gone down for a few moments, he reappeared next to us, motioning to follow him. Poor Darrell looked confused (he thought we were being told to leave), so I yanked Darrell out of his seat and shoved him down the stairs (stumble, stumble, crash, ooof )! Our wonderful usher friend had to make sure the VIP (high roller comp'ed) seats were not going to be used that night, and we ended up being led to the second row, center.....WOW, what seats! I couldn't slip a $20 bill into our usher's jacket pocket fast enough, I was sooooo happy!
I have done this at almost every show we have been to in LV and never been disappointing. If you try it, please remember to be discrete, polite and thankful for what they get you (a genuine smile goes a long way in this town).
Not sure if this will work for Mama Mia as the show is solidly selling out for it's remaining months...there may not be seats to move to!
Good luck whatever you decide, as Mama Mia is a GREAT show!

Tammy & (black & blue) Darrell