Half way through doing the loom I got so hacked off with the standard BSA wiring diagram that I drew my own, along the lines of the Japanese ones I'm used to. This at least shows roughly where the different electrical components are on the bike.
Drawing a wiring diagram is harder than it looks but as far as I can tell this is electrically identical to the original, and to what is on the bike now. It certainly all seems to work (bar the charging system – some rear mudguard paintwork issues mean I've not had it running yet).
The red letters denote deviations from the standard BSA colours. There were a few decent bits of wire on the bike which served peripheral items, and as long as you leave a record to show the next owner what's what then I reckon you've discharged your social responsibility. Black dots show a connector join with different coloured wire either side.
The ignition switch is non standard and doesn't make the connections of the original, so the quaint 'emergency' position, intended to allow the motor to fire with a flat battery, is redundant on this bike. My justification for this act of historical auto electrical vandalism is that I don't plan to leave the lights on all night.
If by some staggeringly small chance anyone would like a civilised wiring diagram for a 1962 BSA A50/A65 running 12V electrics, here it is.
STOP PRESS: This diagram is now available in colour on this blog, Jan 2010. And in a further improved version in Jan 2012 here: http://oldbikehack.blogspot.com/2012/01/bsa-a50a65-wiring-diagram.html