mapping single installs for sake of cookies / single user login?

Could this plugin be used to take advantage of the cookie syncing on single installs (i.e., not WPMU / multisite)?

I have three installs (main and two sub sites on subdomains) currently running off one user tables in a shared database. I implemented a mod to try to get single user login (i.e., login in one of the three sites would automatically login the user in the other two, while logging out in one would log out in all), but while it works for the sub sites, it doesn't for the sub sites to main site.

How might this plugin play with this configuration? Would it work for what I need, and if so, what would be the settings?