Have you looked for the user creation query that is run when a user signs up to xf? you can simply run this alongside your own registration system I would imagine, so it inserts the same data into both tables, or you could use the same user table for each script?
Yeah, user registration should be easy...Disabling logins in XF shouldn't be too hard, though I haven't had the chance too look at the source yet. Being able to log users into XF when logging into my site is what might be a bit harder.
I'm not having any specific problems yet, I'm just asking whether someone has done this before, if they have any suggestions
Edit: thanks for the link, Brogan, didn't think of looking there