Sorry - I've started obtuse, so I'll continue...

Surely to make use of the username you would have to have access to the main entrance db. If you have their trust then you could just explain the problem to them and ask them to place a cookie with the info you need on entry to the main domain. If you don't have access to the db are you just using the username as a label to bunch multiple requests together? In which case you could just assign your own label and set the cookie seamlessly - the same user will then be recognised on entry.

Just trying to get a handle on what's going on here...