domain mapping temporary cookie

lets say;
mainblog.com is main wpmu site
example.com is domain mapped of example.mainblog.com site.

i want to give mainblog.com cookies to example.com for commenting. that can be temporary, like expire 10min later or something.

why i want it? i want to use domain mapping some blogs but i dont want my visitors need to login every mapped domain to comment.

how can i do that?