Once the user is logged in they're no different then any other DNN user. Is this something you need to do on the fly as it happens or you just need to compile the information every so often?

If you are talking about custom reporting per user, we handle this in the report not DNN.  For example, if a user hits a report asking for what items we have that belong to him, the report will grab his user context (his Windows login not DNN), the stored procedure for the report will take his user context and compare it to a lookup table that sees what customer accounts he belongs to.  Those accounts are passed as a parameter to the report and he only gets results for those accounts.  That way we can use 1 report for all of our customers and they only get data that applies to them.  If the user does not have any associated customer accounts in the lookup table, then obviously they will only get a blank report. 

We are using Reporting Services with DNN extensively and in a variety of ways to support a wide customer base.  If this isn't specifically what you were looking for I'm sure I can help with another approach.

Scott