Thank you for your suggestions, Neil. I'll take these in reverse order.
I'm not technical enough to write custom code or interpret FFT data, and anyway shouldn't need to for something as basic as reading off a standard metric from the main display.
There are dozens of graphics configuration settings for spectrum and waterfall, including refresh rate and averaging (though not anything cleverer than on/off). The S-Meter is the poor relation, with only a limited rise and fall time range of control (that I interpret as) to/from the peak reading: fine for peak estimation and useless for the average.
Spectrum (not waterfall) averaging seems to me to be a purely graphical process that hardly (or maybe not at all) affects the S-Meter reading. Thankfully. Refresh rate *does* reduce the S-Meter sampling frequency, but that's it - no averaging, it simply hides more data.
The IF spectrum window marker values are scaled differently from those in the main window, and I've no idea how to compensate for that.
A few additional points:
* Using markers is all very well for an in-depth investigation of one or a few signals, but has too much operational overhead for routine listening, measurement and reporting. Part of that arises from having to switch cursor/click behaviour between two functional modes.
* There *may* be work-arounds that could come close to what I am asking for, but (by the looks of things) would involve extra work and probably some mental arithmetic or worse. In principle an averaging S-Meter should be easy to implement and give the desired result in the right place, in the right format, and with no operational overhead.
* I quite understand that my request may not resonate with radio amateurs (who are possibly the majority user base) as their needs seem to be quite different from the SWL community, and the even smaller number who want to get "down and dirty" with fancy measurements.
* All I can do is state my point of view on a modification that would be a great help in my tiny world, without any expectation of sympathy or satisfaction.