Can I ruin my UV?
Ya know what dude. *I think *if you want a fixed bride 7 string, you're farther ahead to sell the UV (even if you like how it looks) and buy an RG with a fixed bridge. *Then with the left over UV money, upgrade the pickups, refinish it, whatever you want, then you get a guitar specifically designed for the purpose you want. *
If you fill the trem cavity, not only are you bastardizing a wicked guitar IMO, but you will have to refinish the whole thing to make it look proper, losing the binding and inlay on the body anyway and it will never be the same *as a true fixed bridge guitar. *Besides, for the amount of work it will be for something that will never be quite right anyway, it's not worth it IMO.
Just my two little pennies.