mazzer,
I think you can do it that way. If the header pipe is strong enough to support the whole assembly, yet flexible enough to allow you to swing it back out of the way a bit, then you have it made.
Keep in mind the difference in doing it that way and removing it completely is only two bolts! Taking it all the way off would allow you to easily clean up the gasket surfaces....real important.
Don't forget there are three alignment rings that the new gasket will have to be slipped over. If I remember correctly, that intake/exhaust gasket isn't one piece either.
I think very little is to be gained by leaving it dangling to do this chore....but then again, I'm not the one doing it!
Stuart