All good points above. Making a 'mobile' (albeit very slowly mobile) mesh point deployment is not supported and roaming can take a few seconds to a minute. So long as you are OK with that, you should be fine. The labor-intensive way to ensure stability is to figure out the midpoint of SNR between the two portals with the point (walk down the tunnel with the mesh point and check the 'meshinfo' at the console, and whatever the midpoint SNR is, set the mesh threshold 6dB above that. That way when the point does roam, you are positive that you are going to be stable on the new mesh portal.
I would definately recommend using the HP501 as a client bridge solution. It's designed for just this exact kind of scenario and can bridge up to 15 wired devices behind it. If you are needing wifi ON the drilling rig, I think you're good with the AP-275s.
Unfortunately, right now you cannot do multiple mesh clusters on IAP, and there is zero mesh configuration settings you can make (mesh link threshold, etc). It may work but I would guess in the middle where it would need to roam, it would not perform well.