I want to back up Mspec's idea.. reverse print so the plexi itself protects the ink.
We pre-treat lexan for reverse print so the UV ink sticks better but do not after coat it in any way. Not sure how well UV ink sticks to plexi as we have not done it before. But the Sericol ze-1000 adhesion promoter is supposed to work with many plastics other than lexan like we use it for.
We've found 3M adhesives, and others, have a reaction to the UV ink from our Mimaki JF-1631 and causes it to de-laminate so the overlam may cause you similar problems. We are still going back and forth with 3M for options on this one.
Yeah you are right if it's cured correctly it is like a brick of ink.. but the customer is paying for the job so you gotta give them what they want! I have a customer that requested a UV protective coating on top of the UV print we did on white sintra.. I said you don't need it but if you want it you'll get it.. I bought cans of spray clear coat and sprayed it after printed and cut!!