Yup, that's the plan. If I can make this work I'll publish the plans here for fun. The Brits made a lot of these tubes, as well as the Russians (for fighter jets with hardened electronics that could survive a nuclear EMT pulse). You can find NOS (New Old Stock) hearing aid tubes/valves on eBay for $5 each or so, but of course there's not a lot of practical uses for them. I think that tube-active guitar pickups are just the home for them.
I officially do not approve feeding the madness. Why not add a LED and tell them it's a tube. :-)
Back a couple decades ago I designed a "DIC" A direct injection cord, that was a phantom powered JFET buffer/preamp built into the barrel of a 1/4" guitar plug. This JFET front end was low noise and high impedance so we could get all the HF content and edge the guitar pickup was capable of, cleanly.
While I was able to get the guitar input clean and accurate, it generally sounded very thin, because the guitar interacts with the guitar amp significantly to develop it's full "sound".
JR