This isn't much of an answer, but at my company I have put together our own Report Builder.
We have about 30 or so Reporting Service reports that our users can access through the web or desktop application. What we wanted to do was give our users the ability to take any given section within those reports and create their own.
If there is a report we have built for them but they don't want to see the graph, they can create the same report with out it. If they want to combine parts from 4 different reports to make one summary report they can drag those sections around on our custom builder and save it.
The report builder I had to put together pulls down all the different sub-reports they have chosen and reads through the XML adding them to a Report Builder Template XML file I have created. I then have to aggregate all the parameters so as to not ask for them more than once (parameter names do have to be unique across all reports if you don't want them aggregated). This new report XML is deployed to the server and the users can access them when ever they want.
I've also given them the ability to create their own cover pages, headers, and footers by dragging text boxes, images, global variables (date ran, created, ran by, page number, etc... anywhere on a blank canvas. I then convert all the items they've drug around and resized on this canvas in to another report XML file and deploy it as a sub-report that they can add to their custom reports.
Yes, this has taken quite a bit of work, but our users love it. We're in the process now of allowing them to create a report with special groupings so the report can be ran at different levels.
So it is possible, but there is no easy answer. =) I'd be glad to give advice to anyone who asks, but a direct copy of the code is a violation of my contract, but I'll do what I can outside of that.