This instructable describes how a repair can be made for less than $15 (depending on what tools and parts you have on hand.
Step 1: Thread Broken Stem
The tire stem of the TPM needs to be threaded. I thought at first that adding outside threads to the unthreaded part of the broken stem would allow me to screw on a new tire stem. I realized after I did this that this approach wouldn't work since the retainer nut needs to fit over the stem. This was a unnecessary sidetrack.
The approach that worked is to tap the inside of the stem and screw in a new threaded tire stem with matching threads.
I had a tap and die set that I bought at Harbor Freight for $8. The best matching tap size was 12/24. The ID of the broken TPM was 0.166".
I cut out the tire stem from an old bicycle tire inner tube. After stripping off the extra rubber, the OD of the unthreaded portion was 0.236". I used my grinder to reduce the diameter of the brass a little bit until the 12/24 die would snugly fit.
I used a little thread locker to make sure the brass part from the tube fit leak-free in the aluminum TPM.
At this point, the assembly could be re-installed in the Tundra wheel.
Step 2: Fake Out Computer
I needed a way to put the repaired TPM under more than 26 psi air pressure.
I took a short section of 4" ABS pipe and glued a cap on one end and a threaded plug on the other. These parts cost me $15. Any similar closed container that you have on hand would work just fine.
I drilled a 0.4"" hole in the ABS cap and fitted the TPM. After pressuring the pipe to 30 PSI the dash board idiot light went out!! I can carry the tube next to the tire jack.
I'm a little worried that this looks like a pipe bomb if I'm ever stopped by the police. I'm thinking of putting a label on the pipe saying "Tire Pressure Monitor Tester". That should cover it.
All this could be avoided if Toyota made the engine computer open to owners without special electronics.