From my limited knowledge! ( I am no expert)
You have to get the cat to heat up really quickly thats the problem with turbo's and emissions. The turbo delay's so much air through the cat takes a long time to heat up thus doesn't do its job. Susposedly it it relatively easy to get it to work once running for a few minutes, its just start up that causes the problem. Some cars recirculate the exhaust gas on startup which heats things up quicker. Doing it legally? I wouldn't even know where to start. I live in CA and I have heard from the off road truck guys that all you need to do it to A: get it to work. B:Go to the State referee and pleed wth him that you do alot of driving and you need the extra power for your long trips etc.
Another thought is maybe running the turbo after the cat??? Its been done and I have seen an Ad for a turbo system for a mustang that goes on at the back of the exhaust system. I imagine it has huge lag but hell it would be fun.