Interesting stuff, DG - thanks for that.
As to adding the extra (variable pot) channel, I would suggest that you contact someone with an oscilloscope and probe the output of the encoder chip. If the 'scope shows at least seven pulses in the frame you will then know if your particular board is indeed capable of six channels (the first pulse being the 20mS sync/frame pulse). If, on the other hand, the 'scope reveals just six pulses then it obviously means that you have yourself a 5-channel only system.
If that makes sense.
Also, if you intend doing the dual-rates hack, then you might want to experiment with the value of the suggested 1K resistors. Try using, say, 470 ohms and see how you get on.
Incidentally, in those links that you sent, I read that some folk are having problems trying to locate a suitably-sized pot for the dual-rates hack. You could try the 16mm ones from esr.co.uk, or maybe even the 5K ones robbed from defunct servos and transmitter gimbals, etc.
You're welcome to loan/use my 'scope if you fancy the drive to Somerset.