Is the ONLY symptom bloating? Just bloating seems odd.
Actually, the bloating is only very slight (no more than you'd see after them having a bigger than normal meal). The primary
symptom is the near-anorexic appearance of the body aft of the stomach, where it seems to totally collapse in on itself. Take a look at that first picture again: it's not that the thinness of the aft body just looks that way in proportion to the bloat, it's that the width of the bloat is only slightly
more than the normal body shape and the tail has collapsed that much!
Coloration stays near-normal up until death, as does appetite. The infected fish will become somewhat more reclusive and skittish. There are no visible symptoms other than the ones mentioned above that I've been able to identify.
The disease first appeared in a long-established tank that hadn't received any new introductions (plant or fish) for at least a few months. That said, I can't be totally sure that it wasn't introduced by something because of the extremely slow progression of the disease: one fish at a time, dies slowly, then a few weeks later another begins to show the same symptoms.
The first platy infected had been in my tank for 2+ years and the second was actually the ~1.5 year-old offspring of some platies I had at the time, so I think I can safely discount them as the original source.
Thanks for the tip on the garlic!