Actualy, it will work. I have played singleplayer Mercnet missions before, they work just fine. It is when you mix online play with the AI mechs when you run into problems.A single player MWM? You mean a mission launched through mercnet that has AI?
As far as I am aware, this won't work.
Check this link here to get all kinds of maps. There are at least three singleplayer missions. One is called "Radar Raid", another called "Darkest Hour", and a third one.
It may just be that the shell thinks all mechs are automaticly user-controlled during online play. In a similar situation, I was the "Partision Contact" mech on the "MCC1" Mercs campaign mission, and the mech seemed to move on it's own without any aid. The only thing I was able to do was fire my weapons.