yeah, you're not likely gonna find a lot of direct ways to accomplish what you want through a feat or a class ability (or at least any I'm aware of), and most ways that will take a lot of time.
The best way I can think of is to talk to your gm about this. I know I'd be down with eventually letting my characters in some of my games work towards gaining some kind of demon template, or at least be treated as one for feats and effects, if they put the time in and worked towards it.
In all honesty, this is likely the best way to go about it anyway, as finding some obscure secret or cheese way of doing this then springing this on your dm is more likely to annoy him than accomplish anything productive (not that I'm claiming that is what you're doing).