[Arketh]

Turns out, there's actually a very easy way to do it, just overload the cookie domain variable with the correct domain name. I did it by calling a custom function to strip the base domain name from $_SERVER[SERVER_NAME] and return just the domain name (ie: xyz.com from qrs.xyz.com) and then setting the cookie domain variable to "." . $domain.

I'm sure if I wanted to spend a bunch of time at it, I could make a nice elegant hack that ties into the AdminCP, but right now I've got other matters on my plate.