even the slightess amount of sealant which goes to the valve can cause serious problems.
A PIA yes, but serious?
How serious can it be? Dismantle the valve, blow out the core and stem, reasemble.
That should be the end of that.
What Tufo should do is put a small hard plastic foil on the inner side of the tyre right at the air outlet of the valve proper.
That should block the sealant at the valve stem. Only inconvenience is that you'd now have to squeeze the tube around the valve during deflation in order to deblock the outlet.
No big deal IMHO. You don't deflate tyres too often and it's just a matter of being aware of the extra manipulation for the process.
In the mean time, whenever you need to deflate the tyre, make sure to hold the valve side much higher than the rest and deflate slowly.
That should keep most of the liquid at the bottom of the tyre.