Thanks xtempo!
In the meantime I got bored yesterday stuck at home off work due to illness so I came up with this translation of the block diagram in the Japanese manual as I was curious about it:
I was a little disappointed to find it appears the 4-channel input (Line 2) bypasses the crossover network section so presumably the crossover frequency control for the sub-woofer and speaker settings like the 'small' and 'large' selections wouldn't function. It pretty much just seems to be a pass-through which splits the rear into 2 and bleeds off a sub-woofer signal from the fronts and centre.
To use the 4-channel input as that, the unit needs to be set to 'surround off', otherwise processing is applied to the front left and right channels only. So for B-mode audio this is what you'd do to apply pro-logic processing, but for A-mode 3+1 surround needs to be turned off to pass the 4 channels through; otherwise you get pro-logic processing of just the front left and right of A-mode and the centre and surround channels are missing.
I had a listen to the sub-woofer out (A-mode in, surround off) plugged through the centre input of the amp and some form of low-pass/high-cut filter seems to be in place - it's definitely not full range. But the crossover setting has no effect as I assumed above, there is no momentary mute when the setting is changed as is the case when pro-logic processing is happening. Changing the speaker size settings does bring a momentary mute in output, though it does not appear to be fully functional - for example turning the centre setting to off does mute the centre speaker, but it is not redistributed to the front left and right.
Still I'm happy with it overall as it does provide a neat way of deriving a sub-woofer output, and adjusting levels for A-mode audio.