The Shiva/Kinetics Fastpath K-STAR software does MacIP with same-net addressing. That is, you can specify an IP address range on the same IPv4 network your Ethernet side is using for your MacIP clients. No ARP or NAT required.
Unfortunately like many others, I've been having a period of turbulence in my professional life that has made most non-trivial hobby projects take a back seat. I have what I believe is working hardware, but I haven't got much further with it than basic bringup. If what you want is a functional AppleTalk router without paying lots of money for it, I'd recommend having a look at @tashtari 's TashRouter, which looks like it's achieving stability and I will probably end up mercilessly cribbing implementation strategies from when I'm less exhausted.