Morgan- I recently played two custom made OM's by Weber... one at a 20" scale and the other at 18" (or maybe it was 18.5...essentially it was a mandola tuned down to OM pitch). Both sounded pretty good...the 20" had more sustain and depth, the 18" was punchy and good for melodic work (and of course the short scale made it easy to reach all sorts of stuff).
I think the tradeoffs here are... if you tune strings low on a smaller instrument, the sound loses body...and you need to be prepared to play on thicker gauge strings in order to get the proper tension and not have a flabby sound.
But yeah, it can be done. But before you take the plunge, I'd encourage you to put a capo on your Freshwater (to imitate the shorter scale) and play it that way for a while to make sure that it provides the necessary relief to your tendonitis.
Otter OM #1
Brian Dean OM #32
Old Wave Mandola #372
Phoenix Neoclassical #256
If you're gonna walk on thin ice, you might as well dance!