My recollection is that Freedom of the Hills lists rapping through nylon slings as standard procedure. True, the melting point of Spectra is lower than tubular webbing, but if the rope remains stationary (i.e, you rap slowly and carefully), the risk is probably (I said PROBABLY) not too great. I have heard of people top-roping off this set-up (doing so ignorantly after someone else had rappelled), and cutting through. Of course, I'd never use such a sling again after pulling the rope through it! But really, we're generally talking about a situation that would only likely occur when bailing off a long alpine route, right?
IIRC, the "Texas Rope Trick" is used by some climbers to rappel off of hanger bolts - that ordinarly are very rappel-unfriendly - without leaving any gear. Perhaps they use nylon slings, but I personally know of a few that use spectra slings; not the Floss, but the standard spectra slings.
Fortis est veritas