I think there is at least one map that features the mercenaries in a limited capacity, but it's the jumping map and fighting is not really the main objective.

Somewhat surprising that not many mod authors have incorporated the mercenaries yet, though I will admit I've not tried every modded map either so perhaps there is more I'm unaware of.

Modders make do with what is available to them. Unless someone knew a secret and did not want to share.