Just got off the phone when my friend Peter from Onterio, and wanted to get this written down because it's a pretty interesting technique.
The idea is to open an SSRS subreport in a new window (it's always possible to do his with CTRL + Click, but Peter needed it to be the main way it worked). The code below is the code that you put in the link window in SSRS to make this work.
I'll also include a version that will send parameters