set_body referes to an overloaded member of PipelineBuilder and it looks like overloaded members can't be used in CEs. The compiler has to say the following about that:

The custom operation 'set_body' refers to a method which is overloaded. The implementations of custom operations may not be overloaded.

Is it possible to just rename the overload that works on strings to something like set_string_body or set_body_from_string?