Thanks. That was the kind of information I had been finding but it was enough to motivate me to keep searching. I found a link that really dives into the details so if anyone else is interested, it is here: https://web.wpi.edu/Pubs/E-project/Avai ... d_2015.pdf
It's actually a term paper of some sort about the possibilities of hacking into the CAN bus via the TPMS receivers. TL;DR, it isn't something you should worry about, at least for the time being.
It did make a few points about the different types of direct sensors that were interesting. Eg, there are both ASK and FSK transmssion techniques. ASK = amplitude shift keying and FSK = frequency shift keying. These are fairly analogous to AM and FM transmission but with only a few separate values instead of a constant waveform, like audio would produce.
Then there is the fact that each mfg can transmit data in any protocol they want. The experimenters had some prior knowledge and were able to identify temperature bits, CRC (error checking) bits, etc. for the specific sensors they used.
So while I know that there are different sensor types out there, the ones I bought were supposed to work with my car and the guys at the tire shop didn't seem to know much about them, just that didn't think they would work. Maybe I'll try one more shop before I give up. It's almost time to put on my winter wheels and they have the original sensors so I won't have to look at the little yellow lights for a least a few months.