Corbin, I think the simple answer is what Ranisha has said – it’s not a matter of there being a way. There are a million THOUSAND ways to bring the guy back. If they so wanted, he could re-enter in the first five minutes of 4×01 and it would fit easily with the mythology of everything they’ve set forth. It’s a matter of there being a will to get him back.
I agree with Ranisha and DSB: there was a way, but there was no will.
Plain and simple, A and E don’t want him back. So every time someone says something like “oh well, they’ll just do this and he’ll be back by S5” I get incredibly angry. They don’t want him back, it’s honestly that plain. If they did, they’d do it. They can magically handwave anything into existence. But they have no will to do it.