This was a long and winding road: I bought my 1nm filter back in 2018 but had no good opportunities to use it ~19 months ago, due to weather. This time, I had several chances, but found myself struggling to find the right parameters for success, battling unknown sources of internal reflections and double images, variable transparency, and wasted opportunities while I iterated and problem-solved.
Last night, I finally got a result, despite poor seeing. I used an C 9.25" SCT, ASI-1600mm with the cooling on, gain jacked up to the max, and 100ms frames. There is no sharpening being applied. I did not use a Barlow, in order to eliminate one possible locus of internal reflections. Maybe I just need a new, factory-clean Barlow rather than the one that I've been using for 4 years now.
That dark spot should be Beta Regio, so I'm delighted to have captured any surface detail. Endless thanks to the people on here who have posted their successes as an inspiration.