Much like the way Excel has the ability to utilize VBA functions. it would be usefull and more conventient for developer if they could include custom functionality/functions in a report rather than use an external assembly.  The steps involved to include/use an external assembly is daunting at best and not at all straight forward.  

Something like you right click the report and one of the menu options is "Add Custom Function".  Or in the expressions dialog box have an option to create/add a custom function.  

You get the point.  I'm sure your team of developers could come up with something.