How often do you have to tweak the pot, just curious?
Actually I upgraded to a better generator a while back, so this unit doesn't see much use anymore. I don't recall tweaking it but once since installing the trimmer 8-9 months ago. The offset on this unit varies by several mV constantly, so I set the trimmer to average it out to zero, and it seems to hold this pretty well, as long as you accept the fact that it's still going to drift a little in the short term.
Other thoughts: Without this mod and with the front-panel offset control disabled, the R922 input is left flapping in the breeze, so the default offset will depend on the DC balance of the final stage. 300mV seems excessive to me, so maybe the complementary output transistors are mis-matched or something? I dunno. I do know the output stage is biased so that it runs pretty hot even at idle. It needs to warm up and stabilize before anything can be tweaked. I used to use this unit to troubleshoot the ACV function of DMMs, and it would need about an hour of on-time before the amplitude would be stable enough for that task.