My Les Paul in the Avatar was chambered after it was made. However it was done by taking about .75cm off the back, hollowing around the mahogany back, then putting a new mahogany 'slice' back. The jont is 'blended' in and is not noticable unles you know it was done.
I had it done at the same time as it was getting a new neck and a full respray, so it ended up being about a thousand quid. I know it is a lot of money, but I think a few people saw it at LGS earlier this year, and probably didn't realise it was not a real one!
I have a guitar with the neck profile that I wanted, it sustains forever and which looks great. The weight is not excessively light, but it was very heavy before the work (it is actually heavier than my 53 Les Paul still).