One of the major new features of the ARCnet 3.80 driver was the addition of "raw mode" support, so that people could write modules to interface Linux-ARCnet with any other ARCnet station. I guess that means I should document some of the programming interfaces.
This is much easier now than in previous versions, due to an almost complete rewrite of the low-level driver code. If your card uses something vaguely similar to a COM90xx chip, then it's almost trivial to add a new driver.
I suggest you base your driver on the com90xx.c code, since that does pretty much everything you could want. It's only about 750 lines in version 3.90 -- and about 350 of those are the auto-probe routine!
Please feel free to volunteer to rewrite the RIM I and com90io drivers for the new version. Especially the com90io driver should be really easy to do, but I don't have enough time (and I don't have a RIM I card to test with, so I'll never be able to do that one).