Trem circuit built and tested. This tremolo uses a classic LDR method for varying output amplitude, a la early Fender amplifiers, avoiding the use of bias modulation on the PA tubes, which has become fashionable in modern tube type guitar amps. I'm not fond of bias modulation, given the added stress on the PA tubes. The LDR solution just seems more elegant.
The trem board was installed in a prototype 35W amplifier driving a pair of 10" Celestion speakers. The final product of this design is intended for a very experienced local guitar player, and he was quite happy with the trem's sound, so I'm calling it "done" for now.
The only issues I found with the trem board design was lack of sufficient clearance around the two mounting screws. I have 3 boards on hand, and when they run out, I'll do another pass on the design to improve the clearance. My only other slight hesitation is that the trem modulation waveform exhibits a bit of flattening at one extreme. Because this waveforms passes through several non-linear elements on its way to varying the amp's output amplitude, it's not clear how much of a problem this is, but it may be worth attempting to make the trem mod waveform more symmetric.