Does such a thing exist?  I have seen a few that use proprietary barrel nuts, but I want to get one that I can swap on and off with my longer rail without dismounting the barrel each time.

how often are you swapping?

I was going to recommend a samson evo plus hacksaw, but the mounting system wouldn't hold up well to repeated removal and reinstallation.  I think that you will find the same to be true with many of the type of rail that use a standard nut as they use tension from the metal flexing to hold the rail on tight and will wear over time.  

Your best bet might be something like the fortis rev, and take a hacksaw to it. Or call fortis and explain what you want, and maybe they have a blem'd one around with the muzzle end being unusable that they could sell you.

Personally I wouldn't be able to handle having the extra handguard around, so it would soon become an upper that I could just swap out, then I would figure that if I have this spare upper It would need to be a new gun.

Obviously!  Why do you think that I want an extra handguard?  

LOL.  

I have been thinking of the "hacksaw" solution.  

See my quandary is that I am making my pistol into an SBR (as soon as my tax stamp is approved) and I love the looks of the "subgun" with the little short handguard, but then I also love the looks of the longer handguards with just a bit of suppressor sticking out.

I might just get a short rail, an extra barrel and upper so I can have both.  Course then I would be tempted to get yet another lower, SBR it, and have the whole shebang.

Samson STAR-4

I know Daniel Defense used to make one if you can find one used.  Odin Works makes a 5.5" keymod one that I really like, but that may be longer than you want.

Ergo z-float pistol rail.