Really interesting papers. Thank you for sharing! Also, I finally meet someone who seems to have a job as cool as mine 
Re UHF performances under the skin, I’m nowhere near the specialist you are on the subject, but two simple facts seem quite clear: the higher the frequency, and the more you work in the far field, the more attenuation you get if the transponder is near or behind wet ionized meat. This is even quite obvious with NFC tags: HF implant performances are rather pathetic compared to similar sized LF implants in-situ.
I’m actually surprised you got 0.8 m out of a UHF transponder on the palate - which essentially means the antenna is surrounded by 2" of flesh in all directions.
As for the mouth being a hostile RF environment, as described in your first paper, it’s only true if you try to go through it. But here’s a fun trick I use all the time: open your mouth, put your car’s remote inside it, face your car, and you’ll be able to unlock it from a lot further away. Those remotes work at 433 MHz, so your open mouth becomes a resonant cavity.
Anyway, apart from performance issues, the other practical factor that made me give up the UHF implant idea is that the working frequencies in Europe and in the US are different. That means if I want to buy hardware from outside Europe, there’s a 50-50 chance I’ll get something with the wrong frequency, or that it won’t be offered with the European frequency. I can foresee right now that it’ll get frustrating in a hurry - especially considering the price of that hardware.
Care to elaborate? I’m curious to know what you mean by that.
can be annoying