Well, you have to use the parse_bbcode function on the custom profile fields on every instance where it appears as an output.
So to give an example.
Here on vb.org we're showing the realname of a user in the postbit, so if i want to allow bbcode parsing there, i would have to manipulate fetch_postbit().
Also the realname is shown on a members profile, so a parse_bbcode call would be needed there as well