Managing Leads that are Existing Customers....HOPELESS?

We have a large customer base (500K+) customers- a mix of company types and individual types. We are at the point where marketing is starting to mine that data for upsell opportunities on other products. So what we do is create a campaign to track that effort. We add them to the campaign.  Then everything falls apart.

Our process is that Opportunities are opened at the 10% state- so there will have been a conversation or two and there is a definite interest.

However, since the customer record for these cross sells are already 100%, my sales reps have no way to track progress pre-opportunity.  I tried creating searches from campaigns, but they had no way to get them off of the list if there was no interest because the campaign field does not track progression of a lead against that campaign, it only tracks literal email responses.  So we have no way to track whether Customer A (for product A) is progressing against Product B sale efforts.  It's killing me.