I actually just acquired a vintage industrial 31-15 specifically for fm quilting. A Juki 'hopping foot' fits the machine (it's a high-shank), and the harp space is probably like 11" - much larger than the typical machine.
I fm quilted a queen-sized quilt on a normal-sized Singer 15-88, and although it was certainly a challenge, it did work. Kinda like wrestling an alligator, but I actually won in this challenge. ;)