Hmmm, I'm not sure there's much more you can do without additional gear. What exactly does your setup look like? It sounds like your only hardware is a cable modem.
It could take years for a driver for that modem to appear (if ever), and as you say, you can't use RJ45 and USB simultaneously. That would mean cutting off net access to all your other machines while the Amiga is online - not very practical. And configuring Miami to access a cable modem directly is a separate (though smaller) challenge.
The hacked WRT54G and ~$20 USB->Ethernet dongle combination is something you could hook up right this instant. It's cheap and since it has additional ports you can hook up all sorts of wired-only computers to it. I've got my 4000T, 4000D, Pegasos, and network printer connected to mine. If you need more ports you can just hang a hub off it.
EDIT: If you can get one of the Macs and the Amiga connected over a wire, you could probably use the Mac as a gateway for the Amiga in the same way you use that one Mac as a gateway for everything else. That might work without additional hardware (except maybe a crossover cable).