They do seem to go to a dull grey from the initial light appearance- yet the heat of a mA or so of HT on the metal cone/crescent must be negligible. Is there some sort of chemical combination/poisoning going on with the base metal?
It's a shame they are short-lived, I'm sure if there was a simple answer, the proverbial army of tinkerers would have found it. At least the low current requirement suits straightforward voltage doubling, heater boosting takes a bit more thought. It rings a bell that some posh sets with variable selectivity only switched eye HT on in "narrow" for tuning, removing it on "fidelity" positions. Perhaps a subtlely hidden switch for show use would eke out these increasingly scarce and pricey beasties.
I have a couple of tired EM81s around, the oven idea is tempting on the grounds that there's not much to lose!
