Hi Pete,
I'd sugest you to use sessions to pass data between WPs and pages.
The session is "invisible", it does the work silently, it doesn't show up in URLs, so no need to reconstruct URL after the users drilled down and they want to go back.
Please let me know how that goes. If you see that there is no way to accomplish your goal using sessions then we can look into the URL options. What do you think?