Is there any reason you couldn't sit down at a table? If you can, why not bring a couple of tabletop / desk mic stands? They're small, only comparatively heavy in the base, and would allow you to record your mic on one channel, theirs on the other, then balance the mix back in the studio.
A couple of SHURE SM58s, and you're ready to roll, seems to me... and maybe for a lot less than expensive lavaliers giving less-than-optimal results.
Unless you're recording in a bathroom, I wouldn't think ambiance even if they're not dead-on the mic would cause too many problems... make sure it's pointed at their mouth when they're in a natural sitting position, and go to it.
...Am I missing something in this process?