Interesting - but obviously inconvenient for you

So rather than it being the piezos themselves that are going bad it's the water molecules created by changes in temperature and humidity causing a short circuit by effectively forming a 'puddle' with impurities from sweat, cigarette smoke, residue on the piezos themselves etc... (or the water wouldn't conduct).

The old quick cure for a damp distributor cap on a car which wouldn't allow a car to start for much the same reasons your piezos seem to be cutting out was indeed to spray it with some WD-40.  However longer term, the residue left by the WD-40 builds up and actually holds the moisture in.  I wouldn't recommend WD-40, primarily for that reason, plus I don't know what it might do to the finish of your guitar.

I don't know exactly what the compounds might be, but there must indeed be some kind of silicon based 'grease' that is non-conducting and highly water repellent, which if applied under completely dry conditions should keep moisture out of the parts where you really don't want it.   I would guess that the bridge assembly would need to be dismantled to a point to apply the grease to any electrical contacts - plus of course the problem might be at the end of the piezo ribbon cable or connector inside the guitar where it joins the main circuit board.

I certainly would NOT recommend that you apply anything like this though yourself without firstly checking the safety of doing so with Line 6 (phone them up) or with your local authorised Line 6 Service Centre who will be able to seek the assurances from Line 6 that it is actually safe to do so.  Whatever happens you don't want any such grease compound to 'escape' and get over your hands whilst playing, but personally I think that something like a silicon grease might well be the answer if approved and applied correctly.

Just some thoughts which might help you find a solution.

PS - You really need to re-post this marked as a Question rather than a Discussion as Line 6 Customer Support's system doesn't flag Discussions for a required response

Good luck

Nick