We use SSRS 2005 and we have a problem when exporting to excel and then trying to sort the data. From what i can tell you have a custom rendering engine for Excel which may solve our problem..
The problem we are trying to eliminate is when sql server reporting services exports a report to excel it will merge some cells in order to keep the format. The problem is that excel will then not let you sort the columns due to these merged cells. Our users often export to excel and then tweak the data.
I have found it quite difficult to find out exactly what your rendering engine offers over the stadard one MS offers.
Have you heard of this issue before? here is a link for more info:
Hope you can help even if it is just to say this isnt possible to fix.
I have attached a sample excel export generated from a report i knocked up using adventure works to replicate the issue.