Many (most?) roll-your-own NAS Linux distributions (and many of the off-brand prebuilt devices that use Linux) include Netatalk, which works fine with 68k Macs with Ethernet. The only issue I can think of is that prior to some System 7 version (which I don't recall off the top of my head) you may have issues mounting or using large volumes. (Either two or four gigs, again I forget.)