If you modded a ProFlex with the elastomer rear "strut" it would not change length much. It could be your tensioning system.
Harleys have been running belt drives for years with 3 inches of suspension travel at least. Belts can handle some slack, just not as much as chains with a spring loaded derailleur ... And you can add an idler roller with a spring to keep the tension constant, if you want to.
I was just offering a rear end example that did not require cutting the rear triangle ... I'll bet there are a lot of older MTB designs where the seat stays or upper suspension "rods" are moveable/removable, making them easy candidates for a belt conversion. Limit the travel and you are good to go
An ingenious person could prolly calc the belt length so that as the rear triangle came into alignment, it would induce correct tension ... And maybe adding a simple roller in the right place would keep the tension as the swing arm moved a bit for better ride ...