I have done it many times. If the guitar is old or has been polished with silicones in it's past give it a wipe with mineral spirits followed by naptha. Use a clean cloth for each you want to throw away the silicone not spread it. Now give the entire surface to be painted a good sanding with scotchbrite cloth to take the shine away and give the lacquer a tooth. That is all you have to do, I find it is not necessary to try and take off the clear coat and you do not have to deal with sand thru. Blow the dust off. One more naptha wipe and you are ready for the tack rag.
Go for it!
OK....my first post on this excellent site after doing some quick browsing.....
Why would you want to put lacquer over a poly finish? Are you going to use a lacquer that has a tinted colour? I've never heard of this before....is it possible to change a guitar's colour this easily?
A budy of mine sprayed black lacquer over a MIM Strat that had an original red poly finish.
Everytime he nicked the guitar (he didn't really didn't baby it... can't blame him) the black lacquer would come right off.
I got the feeling if I dug my fingernail hard enough into the finish, it would come off. If you have a decent body, and want to do a nice job, strip the poly off to bare wood. I think you will be much happier with the end result.