Ah, now I see.
I think outright refusals on the basis of no intent to return are relatively few - although they can ask for more info. So if you mean that your plan is to move to join your spouse, I think that is a decent approach. If you keep some ties in Canada you should be okay.
I think they would need to issue a PFL if they did wish to refuse on that basis, at which point you could return and advise of change of address. (More typical is they just ask for more info about your plan to return).
But I do not know the technical question of whether they can on appeal disregard. I warn I think this is overthinking - but I did read a case where the sponsor had returned to Canada specifically for the appeal and the IAD didn't accept that evidence; but the other aspects of the case were all rather extreme and the spouse who returned had clearly returned only for the appeal and was sort of living out of a suitcase. And I don't think that case turned on the 'intent to return' part, it was just one aspect, possibly just as H&C [meaning the part where they considered whether H&C outweighed all the other stuff that had been established]. In other words, I don't think they would routinely do this nor have much of a leg to stand on except in quite extreme cases.
I doubt they have plans or contingency plans about your case. It's mostly just a bureaucracy.