I just looked at my Dad's '83 Shadow (It has leaky forks that need boots), and I thought of something. Does anyone make a cap with a tire valve to pressurize the forks, or does it require an air bladder or some other parts? Is it practical? I gather that someone here made the transition. I remember that someone in my Rotella fork oil thread challenged me to find a way to keep from having to overfill the forks in order to eliminate bottoming out. This may be the answer he was looking for...

The gap between the cap and the oil level already acts as an air pressure reservoir. Would it be so hard to pump some more air in there?

Sealing oil and sealing air, totally different problems.
How many square inches would be involved in the diameter of the smaller diameter section? Six square inches? What pressure would be needed to hold up the bike? And passenger?
I've never seen a pneumatic suspension on bike forks. Seen it on a swingarm, and air up shocks on a pick-up,
Not gonna take much pitting to cause a problem. Forks take a beating.
You would be the trailblazer, AFAIK.  Sounds good, go get 'em!

some touring bikes have air adjustable forks,(Gold Wings, XS 1100s GS 850s and I think 1100s) but they come from the factory that way, you'd need to have a look at them and see how they do it, and copy the system
I doubt it'd be easy, and it FOR SURE wouldn't be cheap