Dynamic System Resolution and Custom Login Modules

I'm trying to achieve the following, and wondered whether anyone could validate that is possible, and possible solutions.

We want to use Dynamic System Resolution to programatically determine which system alias (and therefore which R3 client) should be returned based on the user. So far so good.

When the user logs in, we want them to be able to specify which R3 client they wish to use whilst logged in to EP. We are considering writing a custom login module to do this, which will strip the client the user wishes to use out of the j_user username, and stores the client value *somewhere* so that the Dynamic System Resolution code can access it and base the system alias it returns on the user's prefered client.

Considering the DSR code only has access to the IUser object it would be handy if our login module could store the prefered client as an attribute in the IUser object. Is it possible to set custom attributes in custom login modules for a given user?

We want to do this to avoid having to have an EP instance per client in a given R3 system, and to avoid duplicating worksets by creating delta linked copies and overriding the client number.

Any suggestions?

Cheers,

Steve