Hi Steve,
This is a tricky one and my first thoughts were that the SLX Provider would prevent you accessing the other datasource.
After speaking to one of our consultants, they vagually remember doing something similar about 2 years ago and instead of including the repot in the reports section of SLX, they used a button on a form to call the report directly. I don't know how/why doing it this way would be any more successful, but it may be worth a go. The downside of course is that you'd need to customise a form...
Also, checkout www.SLXDeveloper.com for this sort of thing as there'll be loads of people who'll be all over this like a rash (if in fact this very question has not been asked before!)
Ben