I thought that a product like that might cause a problem with the foam backing, but a chair (and a car seat) uses a similar material.  There are only about 8 small screws holding the fabric cover on, so that is not a big deal either.

Hi Andrew.  Funny that you should mention rain.  Heard Angela and Jennifer Chun last week at a world premiere of a dance choreographed by Jonah Bokaer (Late Nights on Air) and I swear their instruments were in Cecilio vinyl cases.  At first I excitedly hoped that the violins were Cecilio as well, since their tone was excellent.  That hope was dashed when I learned via the internet that they play valuable antique instruments.  Perhaps that night?

All of their album covers seems to have photographs of them walking in the rain with their respective violin cases exposed to the full down pour (more credence that they might be Cecilio violins).  Hope springs eternal.

A vinyl covered case, as well as fiberglass and carbon fiber, should protect from rain quite well.  I have seen photos of expensive cases with a removable cloth envelope.  It would be fairly inexpensive to provide a cover of gortex cloth and taped seams to provide excellent water proofing.  And your idea of a plastic bin bag should also work.

Goretex sheds water but passes water vapor.  I use it for diy violin case humidifiers.  It may also be possible to use a waterproofing spray commonly used on outdoor jackets.