After many such ill-fated escort missions, I slowly learned (I think) that the folks you're trying to guide to the portal kind of intuitively know its general direction. They will charge to their deaths if they see monsters, but I believe that the main reason they seem to deviate from your course aside from suicide by monster is that you're not headed toward the portal. So what you can do is use them as a kind of divining rod.
Still very hit or miss, but since the rewards are potentially huge for success, it seems fair. The stakes remind me of portals in Crawl - dangerous, uncertain, but potentially very rewarding.
EDIT: Man, you know, the more I play it, the more it's not my kind of game. Every time I fight something that poses a challenge, I have to queue up all this stuff... and since there's no need to eat, sleep, or drink, and since wandering monsters are really a nonissue, there's no reason not to wait until the cooldown times out between battles. Gah... it's maddening to me when the most cautious playstyle is so gleefully rewarded. If I'm not feeling the pressure, what's the point? Alright, that's all I've got to say about it.