Creating links in admin pages

I am either going to set up multiple wordpress installs or use wp-hive to run several blogs on the same install. Either way, I remember at some point being told that it would be possible to add links to those other blog admin sections from the the main blog’s admin section. Is this possible from editing some php or is this getting into custom plugin territory?

To better explain, i’m setting up a site for someone else. They want multiple blogs running at site.com and members.site.com, but it would be more convenient if I set up both of those sites with the exact same login information and somehow provided a way for them to move easily between one backend and the other (they would of course have to log in each time, but they wouldn’t have to log into one of the sites very often.) I’m just trying to make it as easy as i can for them.