Just a quick note, more to myself than anyone else, but I wasn't able to easily find this information online, trolling through the OpenWRT and Ubiquiti forums.
I wanted to try booting different kernels on the device before committing to flashing, so that all I had to do was reboot the board and everything would come back up as normal. It seems obvious that this should be possible, and with a bit of digging around in RedBoot docs, trial and error, and a few lucky guesses I was able to figure out the incantation to make RedBoot load an uncompressed linux kernel (ELF) and boot it:
load -m tftp -h 192.168.1.1 openwrt-ar71xx-generic-vmlinux.elf exec -c "board=UBNT-RSPRO panic=1"
Note that this does not change the root filesystem, so if your modules aren't compatible, this won't work, but I suspect if you use an embedded initrd you can run the system entirely out of RAM without touching flash.