I don't think Wobbuffet's speed is really important at all since it always attacks last, so would it be sensible to let it hold a Macho Brace to double the amount of EVs it earns during training battles?

Why not? Wobbuffet is going to attack last anyway, like you said.

In fact, I don't really care about Speed during training, because it's training, period. As long as the Pokemon is going to survive, there's nothing wrong with taking a few hits

Wobbuffet is certainly one worth raising, just get a calm nature