It is very hard to use the available objects (like sampler) in a BF sampler or Pre|Post processors. It would be nice to add some support for code completion or some kind of Intellisense support.
Lowering priority as it's rather lot of work for not much ROI. But this could be a starter if Modified BSD license is compatible: http://fifesoft.com/rsyntaxtextarea/
I think JMeter is not an editor, and there are many good one available in Eclipse, Netbeans, ... Closing as it's really too much work and not really related to Testing.
This issue has been migrated to GitHub: https://github.com/apache/jmeter/issues/2832