It's possible to use custom code in the reports in a Tier 1 environment, however, in the Tier 2+ environments, using the same custom code you receive the following error:

"The report server has RDLSandboxing enabled and the report contains custom code. Remove the code element from the report definition"

As workaround is possible to remove the code and use datasets, however, it should be very interesting to find a way to allow this custom code in the future.