heh that's a good question. I don't know the answer.
however juggernauts themselves may currently be buggy - there was a report that they were not spreading dominion unless they were given the gift of reason. whether this was addressed in 2.12 or not i don't know, but I think not.
Archaeolept is correct, as of now juggernauts don't work right for anybody, much less Mictlan.
Once they are fixed, my guess would be (but it is just a guess) that they would spread dominion just as well as the real Mictlan prophet, which is to say, not real well, but better than nothing.
actually, the mictlan prophet does zip. only the god spreads at 1/3 the rate of a normal pretender.
if they're hard-coded to act as a prophet or as a temple, then they wouldn't work for mictlan. if they're coded to spread a set amount of dominion, then maybe they would.