SUMMARY: Long-time Pairs Relay rule apparently isn't.
In USDAA pairs relay, if there are an odd number of dogs, one of the dogs who is already entered in another pair can run a second time (as the "accommodating dog") so that the leftover dog can compete. The rules in Chapter 6 for Relay state that this additional run is nonqualifying for the accommodating dog, which is fine.
However, although everyone knows that the accommodating dog must run the opposite half of the course from what they run with their original team, in fact that rule doesn't seem to be in the rulebook. I asked a bunch of folks about it after trying to explain the rules to a non-USDAA person and finding that I couldn't find it anywhere. Don't it make you wonder what other "rules" are really just word of mouth?
I shouldn't have said anything and just used it as a secret weapon at some future time: "No, I'm not going to run a different half; tell me where in the rules it says that I have to?" But noooooo--I had to blab my question to everyone and now JB says he's going to bring it up to the rules committee to make sure that it gets into the next rules book. Ah, well, better luck next omission.