Doing a live take and then using it as the backing track to do overdubs is an option, but it allows no wiggle room for mistakes, whereas live you can compensate for each other's little tempo changes, etc.
If you want to record it live, you can:
Run amps/pedals/DIs, directly into put headphones on everyone (drums being the only "live" miked instrument), and record that way (if you go the DI route, you can add effects later, and/or even run them through the aux loop so everyone can hear them as the "live" recording takes place).
or:
Wheel your amps into different, separate rooms, miked, and put headphones on everyone. This will minimize bleed-induced cacophony. It will require some long cords, but allow you to mike your amps for a big live sound without the small room muddle.
Either way, I would have to think that the harp and the drums will absolutely have to be in separate rooms. I just can't imagine that miking them both in the same room could possibly sound reasonable. Even if the harp has something like piezo pickups, your results would probably be much better if you miked it acoustically...
Just a couple of options that come to mind--I'm no expert by any means...
