Not sure boots over shoes is really an acceptable answer. Someone said that an eraser can rub the oxidized layer off of metal, and once the oxidized layer is gone, your in trouble in a high pressure O2 environment. It does NOT have to be a spark; it can just be exposing unoxidized metal to the high pressure O2. Similarly, glue-ons aren’t the solution if the glue is reactive to pure O2 (or the shoe material, for that matter).
Just as there are ways to protect humans who can’t be put in a chamber without some metal implement, there should be ways to accommodate horses who’s injuries are such that they cannot be without some kind of support on their feet.
But also consider that a horse in residence at one of these rehab facilities–assuming no extreme foot pathology–can probably be maintained without exposure to footing that would dammage sensitive feet. Going barefoot to increase safety inside the treatment chamber isn’t out of the question for the horse with a medical issue.
The horses getting this therapy while still in work, though–that’s a tough issue. I would anticipate some rapid development in hoof boot choices in the near future … Either in place of shoes, or as substantial shoe coverings.