Just to put the cat among the pigeons a bit........Nepalese weavers have a technique whereby they can add another 2 harnesses to their usual 2 harness looms, in the middle of a warp. It suddenly struck me that this is a technique that rigid heddlers can use! I don't have the book handy to see exactly what twill weaves they use this with, but commonsense dictates that the new warps should go through the slots of the original heddle.
Since I have my knitters loom already warped up with the 2 heddle block on it, I shall go and warp up the second heddle to try this out and report back tomorrow on the success (or not) of this idea ( its evening in Australia so you will have to be patient!). At the moment I can't see any reason why in theory it shouldn't work, but perhaps I'm over optimistic and need to find the instructions, lol!
Cheers, Caroline