For the tube stem? I probably would've used some RTV on it if I had any, but the stuff I thought I had was all solidified; had planned on using it to seal the hub up for the ATF experiment.
So far, at least just sitting there, the heatshrink-over-patched-valve-stem repair is still holding 55PSI, after a bit more than 24 hours. Maybe 28 by now. Has to survive riding on it with all the pressure changes caused by bumps and potholes and whatnot, before I would consider it a success, though, and I have to resolve the ATF leaking on to the rim brakes before I can do a ride with it.
If it does work, I am likely to do something like you're describing on *all* the valve stems, to hopefully prevent a stem blowout in the first place, as I strongly suspect that what is happening is the extreme pressure changes from hitting bumps/holes with all the weight of these bikes on them is causing the stems to balloon out at any tiny defects in the bond between brass and rubber, and at some point it is mroe than the rubber can stretch...so, BANG.
If that is indeed the failure mechanism, then the heatshrink will prevent the ballooning, and thus the rupture. It may not be as successful at holding a patch on airtight after a rupture has already occured, but we'll see on that, too, with this test wheel.
Note that most of my more recent (last 2-3 years) stem failures have been on CrazyBike2's rear wheel, which is unsuspended *and* carries at least 3/4 of the bike's dry weight currently, and previously had anywhere from 1/2 o 2/3 of it, depending on battery and motor placment in various experiments. Also, back when DGA was unpowered (2005-2007/8) and then SLA-frictiondrive powered, it was about equal failures on both front and rear, regardless of weight distribution (which varied a lot as I experimented with drive systems and battery placement, as well as carrying cargo). On pedal-only bikes prior to either of these bikes, I saw about equal failures of front/rear there, too, AFAICR, but cargo would have been in baskets both front and rear, over the wheels adn to the sides of them (like the Columbia bike pic shows at the very start of this thread, before it became DayGlo Avenger).
Oh--I also did a little test overnight: I took some of the dried-up rubber cement off the outside of the lid and bottle, and left it soaking in a capful of ATF. Today before I left for work, about 13 hours after doing that, there was only liquid in the cap. ATF most definitely completely dissolves rubber cement, even if it's been dry for months!
So don't use it to seal an ATF-filled motor.