Hi again, Matt.
Here's one of the things I'm seeing:
There are a few portable ventilators out there designed to work at hyperbaric pressures. They're hard to find but they are there. I think the best bet in this case would be pneumatically controlled one. Requires no electricity but runs off of a gas sourse; it's all mechanical.
I see an umbilical running from the source gas thru a sealed umbilical inlet in an oversized drysuit in which also is you, Matt. The vent can be held in something of a backpack. Inside the suit the source gas would be running directly to the vent...therefore no freeflowing breathing gas in the suit. From the vent, tubing to the trach via a one-way valve and wye connector. The trach connection can easily be made to be low profile to minimize interference with the neck seal and probably elimitate it altogether. The expiratory limb would run back to the umbilical inlet in the suit where another one-valve would vent expired gas to the water. This valve would be set to 5 or slightly higher cmH2O- your PEEP level currently.
A modified regulator would be needed to source the ventilator with the necessary pressure, usually in the neighborhood of 50 psi.