If you move a Sim (any Sim from a different household or from the bin) into the house and then back out, the missing husband should reappear by the mailbox. At least, this works for fixing babies and toddlers that disappear in this way.
The only problem with that is that it would add money to the household defeating the purpose of a legacy challenge. I sorted it out by moving him and the wife out by edit town and placing them in their own house and he reappeared there. I never got to give them jobs or anything so they'll probably get destroyed by the story progression but whatever, he annoyed me so much I'd kill him myself.
Thanks for the suggestions everyone.