Well, to start with, I can't find anything in any books that give an indication of how the Capture lamp is enabled. The N/L ARM light comes on just to show it has been selected. I am not sure if the deviation signals have to be sensed by the A/P processor or not. At this point, I think the Nav valid signals enable it. If you look at the lower right hand corner of Fig 3-6, sheet 4, you will see the Copilots's Nav/ILS Flag Driver. AYW1-107A22 goes from pin 6 up to the AP Processor Nav Valid #2. Looking at the diagram, I don't see a SCNS input.
In the 22GS, FO-5, sheet 9 doesn't show it either. BTW, these foldouts are pretty decent to see signal paths through all the different relays.
I am checking with someone that knows SCNS very well to see if he has any input, but I will keep looking. Please keep in mind I've been retired for 8 years, so it may take me longer than it used to.