Ah hell, I should have recognized your call. I think we had some correspondence a couple years ago too; I was KF5SAB then.
Congratulations on the progress with the new Charlie board; your project is somewhat difficult to keep up with as I do not have a lot of time to dig deep into the github commits which is the only place i have really found to see the nitty gritty details.
"cheap" ET is still relatively new -- it's only hitting consumer LTE devices now in 3rd generation hardware. iPhone 6 / Galaxy 5 are the first phones in those two lineages to use it for instance. Open designs for amateur radio are nonexistent. The only one I have ever even seen which comes close is Helmut's paper. He is the correct guy to ask. But that is only a very basic design for the power modulator. I understand his PA in this design was an LDMOS running in class E.
You are right about the ET modulation being relatively straightforward. Recent versions of openHPSDR's PowerSDR (and the corresponding radio firmware) actually support ET out of the box; they gnerate a modified signal for the PA signal with the DAC and a power supply modulating signal (high frequency PWM) on one of the J16 IO output pins. The power value is obviously the scalar sum of the I and Q vectors, and I believe the DAC output is somehow normalized to full scale so that it will drive a class E PA successfully.
Doing this at HF may be significantly different (and easier) than doing this across 50-1000mhz though. But feeding a class E PA with a modulated power signal based on Helmut's basic design might be enough to do some very worthwhile experiments. A PA that works across that whole range is probably enough of a challenge!
If you want to discuss further, feel free to email me --- my call at arrl.net