Easiest... try to find another taper in the area with a 4, 6, 8 channel deck and invite him to join you. Like he said, run the stage mics through the house snake, so they all meet in the back.
Better... find someone with more channels, like 12 or 16... 4 vocal mics + 4 instruments + SBD matrix + 4 room mics... or something like that. There are lots of computer interfaces that will handle that easily, as well is bigger gear like an HD24. It's amazing how much better you can do with individuals channels than you can do with a simple AUD + SBD.
If all else fails, run what you've got... your good rig on stage lip, and put the mics a little higher, like waist high on the musicians and closer, so they are getting a mixture of direct and monitors.
Check this one out.... that what he did and it came out pretty damn good.
http://archive.org/details/AOD2010-02-28.early.baker.xy.stagelipThen scrape up a couple of other sets of gear, and run your 6 channels. Chances are you can find 2AUD + 2 SBD, and align them, and you'll decide you can live without the 2nd pair of auds. But run both sets of auds, because you don't know for sure which will be the best 2 until you get home.