I know that Micronik used to make an A500 tower kit, with a busboard which had Zorro slots, a kick switcher and an A2000 CPU slot.
You might be able to find one of those floating around and put a Zorro ethernet card in it.
Ebay? Amibench?
[edit]
Oooh, i forgot. Amitrix do AmigaLink (
here) which is similar to ethernet, ut significantly slower. Works though.
And IIRC, in theory, a scsi network is possible, never seen such a thing implemented on the A500 though. I think the Siamese System used one.
[edit again]
DOH! It would appear that AmigaLink is no longer available.
However,
aminet/hard/hack/AddZorro2slot.lha has details on how to add a Z2 slot to the A500 with a passthrough for your SCSI interface...
