I attended the 2-day PLAIN Plain Language conference in Sydney last week. It was excellent. While many people think of plain language as simple, jargon-free language, it is so much more than that. Plain language incorporates insights from a broad range of fields including information design.
The best definitions for plain language place the reader, or document user, at the centre. A document is in plain language if the user can use it in the way intended by the document writer or designer.
Therefore, plain language research is first and foremost research about the document user.