The only way i think you can have this work out is if you have a FULL custom exhaust made. This means the Y pipe BEFORE the cat so when its open you will have no cat in the way. Weld the ECS/y pipe directly to the cat. then you will have to have a custom cat back fabricated because the length will be too long in the ecs/cat set up. you will then have to find a way to run all the O2 wiring correctly which may be difficult due to wire length limitations. That will work just fine.
P.S. even if you do this i dont think a Y pipe is legal
I can check if you want tho'