No, there is no easy way to do that with Poseidon for 68k. MorphOS would go through the mount.library, but on 68k, it relies on custom filesystem detection routines inside the massstorage.class -- which doesn't have support for the rather exotic HFS formatted media. Sorry.
Well, that's a bummer. This is a rather exotic case for me as I don't normally deal with Mac-formatted media (though I do have a MAC0: floppy device always available,) so using a special mountlist for the Zip drive (or others) isn't such a horrid thing.
I'll try it on MorphOS, as well, to see how that works for me. I'm slowly getting used to it though my A4000D is still my main machine. (Also, thanks for recognizing that I'm on a 68k machine -- I forget that 68k isn't the only game in town these days.)