i was at your point a few years ago. one of the first jobs I tried to tackle after joining the forums. basically, i did not find a way to do it. eventually i had to have the car towed to a muffler shop have the exhaust dropped and the gasket replaced (with the one i bought/supplied).
if you find a way to do this, then post back with how b/c even the more experienced members all agree that dropping the stupid Y-pipe is a must.
Well, job done. I ended up cutting the cat and welding it later. I got the front flange bolts loose but couldn't get the rears to break free. So one good cut with a sawzall and it was off. Getting to the top of the pipe with the welder was quite a challenge. I'm pretty sure there's a slight bit of an exhaust leak there but it's quiet as it was before the cut as far as I can tell.
During the gasket replacement I really don't think it'd be possible because the gasket needs to go around the crankshaft. But anything's possible I guess. It'd take some real skill though! and some crafty use of bailing wire or some other means to hold the gasket up while fishing it through. For me, getting the Y pipe off was impossible (at least with my tools and on this car). So the sawzall and welder were the best way out.