according to this BOB post it is possible to create WebI reports from stored procedures. The original poster used XI R2 however later posters appear to have achieved it in 3.x
Having said that I have been attempting to both build universes and reports using stored procedures (MS SQL Server 2000 and 2005) in XI 3.1 without much luck although I can get DeskI reports to work intermittently.
I would be very interested to hear how you get on.
clg 
(BOB member since 2009-09-23)