If you're running OS 8.6 or later you can do Carbon development. That gives you access to all the tasty bundle goodness people are using here. 8.1 gives you access to some of Carbon, but I'm not quite sure how much; whether CoreFoundation is included. My recollection is yes, but I could be wrong.

ANSI C's fopen call will get you access to the data fork of a file with a relative path if that's all you want. You'll still have to do bundle stuff when you port, but it'd be a solution for the moment.

There was a "standard" way of doing this sort of thing which involved passing a relative path and 0 volume & other flags to FSMakeSpec or something -- but I never did it myself. Somebody else?