Sounds to me like you need a quality CTIS. The military use a Central Tire Inflation System on many of their vehicles. There are probably even commercial available kits out there too.

It's a major modification though so I'm sure it's just a pipe dream.

Think about it though: Approach the trail head & just set the desired psi on the dash. The system automatically airs down all tires to the desired pressure ... AND ... maintains that pressure under all wheeling conditions. Approach the trail end & just readjust the psi for road use & the system has you aired back up as soon as rubber meets asphalt.