Hellfire Warlock is a pretty intense boost though, despite being only three levels long. And the obvious alternate additions, Eldritch Theurge and Eldritch Disciple, have some very cool but extremely open-ended class features that I imagine would be an unholy pain to code.
Still, you'd need a way to mitigate the Con damage. Since binding and incarnum (the two primary pnp methods) aren't around, I'm not sure how useful it'd be.