That would be impossible because, unless the tank vent is blocked, the pumpout would only pull in air through the vent line. But even if it did pull air through the toilet, it would be pulling in the the direction that the joker valve is aimed, so it could not damage it.
Replacing the joker valve--which should be done annually as routine preventive maintenance anyway--blocks the backflow, but only till the new joker valve becomes slightly worn...so it's just a band-aid, not the real cure.
The real cure: learn to flush longer in the dry mode. Any toilet that's working anywhere near factory specs can move bowl contents up to 6 linear feet or 4 vertical feet in the dry mode...so if you don't leave anything in the line, nothing can run back into the toilet.