correct me if I'm wrong, but couldn't you rent some kind of multitrack computer interface for a day, get all the tracks in sync in the digital realm, and then mix to taste later?
This is good advice. They day rental on an ADAT would sure be better than all the mess of trying to simulate a multi-track environment.
I use a single ADAT to record my band live, then I go lightpipe from the ADAT to my Digidesign 002R for purposes of archiving/mastering/editing, etc. Before this, I think I tried just about every ghetto method of multitracking out there.
ADATs are sort of out of style now since ProTools hardware is so cheap. You can get one for $250 on the used market, but you should leave some budget for repairs. (They are tape devices, after all, and are a bit tempermental.) The Digidesign 001 goes for $750 used, which is probably the next best step up.
There are some four to six channel cards out there cheap if you're trying to just scrape by. However, I've found that scraping by with multitrack can be a lot harder than scraping by with stereo. A lot of low-end computer based systems can be *really* frustrating. Heck, even ProTools can be a pain sometimes, but it's rock solid compared to most.
If you just wanted to experiment a bit and learn, you might consider some of the older "Portastudio"-style systems, like the Boss BR-8, for example. It only does two tracks simutaneously, but it has some neat features, and you can get free software to let you read the WAVs directly from the Zip-discs it records on, which means you can get them into ProTools (or Cubase) pretty easily. They're about $200-$300 on the used market.
Honestly, it sounds like your best purchase bet for multichannel recording on multitrack is some kind of interface for your Cubase equipped Macs. For the truly low $$ route, follow the above advice and rent.
Good luck -- it's a lot of fun.
Chuck