I've been reading a bit more of the service manual.
There's (yet) another variant that has RGB inputs on separate BNCs. These go through the PAL decoder IC, which amplifies them, meaning that these are "normal" 700mV signals.
Retrofitting these looks to be quite easy, as it's just wiring and sockets to add (the rear panel is pre-punched under the label). Tempting to consider a Scart socket
Sync on green is supported by moving a jumper on the decoder board (TL1).
Studying the diagrams, for those sets with PAL decoders, it looks like it might be possible to re-wire the connector that plugs into TL103 to get analogue rather than TTL in via the DIN socket, but without testing it I wouldn't guarantee it. I also don't see the point as it's 0-4V. It's not mentioned as a possibility for those sets that have the PAL decoder - only those without. So instead, simply go in to the PAL decoder IC as described above. Sets that have all 3 options use a 3-position centre-off toggle switch to choose between TTL RGB, linear RGB and PAL - all the logic for this - such that it is - is already present.
Either way, should you come across one of these that is composite only, adding TTL or linear RGB is trivial.
Happily, I can add that the manual I'm looking at
does cover the Spectrum interface, amongst others. That's the first time I've seen those detailed, and is very lucky, given that this particular manual was about to be recycled (we haven't used them for quite a while). As and when I eventually get some quality time with the scanner, I'll make them available for all to see. Just don't hold your breath
Cheers,
Mark