| All known modern TX, except AM, have diodes in the charging input
Not true.
Many have fuses (often self-resetting poly-fuses) rather than diodes, and these have no problems with charging or discharging through the charge jack.
Examples include the Futaba 9C and Multiplex Evo 9. (The Evo 9 actually puts the fuse in the battery pack itself, so even if you take it out you're charging through the fuse.)
| which must be bypassed to allow fast charging.
Not true.
Some peak chargers (let's say peak charger rather than the more ambiguous `fast charger') have no problems peak charging through a diode.
My Triton has no problems charging through a diode.
My Astroflight 110D cannot charge through a diode.
If you hook up a multimeter to charge jack on a diode protected TX, you will read zero volts -- that is normal. A peak charger can get around this by attempting to push a tiny charge current through -- like 1 mA -- and measuring the voltage. Using that, it can make sure the battery is there and the polarity is correct. During normal charging, it simply reads the voltage required to push X amps through
-- the diode doesn't hurt things there either (it just increases the voltage needed by a small, mostly fixed amount), and the small voltage drop that indicates full battery is just as easily seen.
| Note: do not charge at greater than 1.2A as TX circuit may be damaged.
Yup, diode or not. The charge jack and traces and such are not meant for high currents. In fact, if there is a diode, I'd suggest not even going that fast -- perhaps 0.6 amps would be better.