A plugin would be fine – just haven’t found one, that suited our needs.
Maybe I should elaborate a bit on the scenario, that we’re considering.
We’re planning a network of sites. Some with paid content. Some with free content.
When a user registers/logs into a site, we want him/her to be logged into all sites and have a profile on all sites. That doesn’t seem to be a problem.
But we also want to present specific content to user, when they enter a new site in the network. An example could that a user, that has registered on site B, enters site A (with paid content) he is presented with special offer, maybe reduced subscription fee, maybe an ad or maybe the user is able to see parts of the content.
Buying a plugin or maybe even having one developed would not be a big obstacle, but we would appreciate opinions on, whether it is doable within reasonable limits – we don’t want to rewrite core or anything like that.