I don't think it would be difficult at all to introduce the Land of Madmen. All you need is a casual conversation with the newly-chatty Sea Folk. Harine busts out and says "Oh, well he could be referring to the Land of Madmen. That is a horrendous place, full of crazed male channelers, avoided by all of the Atha'an Miere." And bam, there it is.

But to then say that Demandred had rustled up a thousand crazy male channelers from there without having time to tell us much about the place - that might be too much of an eye-roller to overlook. Although I will say it wouldn't be without fantasy precedent. Those dudes from Near Harad and their buddies who showed up on Sauron's team in LotR weren't explained very much, not in the main trilogy anyway. Just whoosh, here are some bad guy allies from the south and Bob's your uncle.