Hi Douglas
I imagine your question is familiar to scores of readers on this list. There are probably many ways of approaching this problem (which might be a very interesting research question in itself). I will try to be short and practical, rather than long and philosophical.

I have just finished defending my thesis and have struggled with this issue the entire time. My thesis is article-based, and in all of the articles as well as in the first version of my "kappa", I wrote only briefly about design part of the work because I deliberately did not define it as a research method. I described it as a design practice, that occured parallel and inbetween the research method phases, and only outlined it shortly. The designing I was doing was based on my personal experience as a designer, it wasn't replicable (if someone else had done it, probably different results would have emerged) and it wasn't very transparent. My way of working wasn't always systematic, and I often changed the question I was working on, like designers tend to do. So it didn't fit very well into my understanding of what research is.

However, one of my supervisors convinced me of the importance of describing design part of the work, as well as the importance of reflecting about this work. Both designing and researching are "methods" simply because they are parts of the work that has been done. So I spent a lot of extra time reading, thinking about and writing up this part for my kappa. I gained a more nyanced view about how they differ, heavily influenced by the work of Rosan Chow. The new version of my kappa is much longer, both background and methods chapters expanded to include more about the designing. I also included a section at the end that is pretty much a reflection about identity, because I found - like Robert Harland who also commented your posting - that this is also a relevant issue in discussing designing in research contexts. 

Now I'm experiencing the consequences for having left the designing process descriptions nearly completely out of my articles. New projects have been started up by other people in my research circles, and my methods are being copied. I realize that these people have nearly completely left design part out too, but it is my own fault. So I have to go in to a lot of meetings and explain what it is that was left out, and help them put the designing back in. 

Good luck with your work.

Sarah Rosenbaum, ph.d.

Designer/senior advisor
Norwegian Knowledge Centre for the Health Services
www.kunnskapssenteret.no
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References:
Chow R. For user study. The implications of Design. Braunschweig: Von der
Hochschule für Bildende Künste Braunschweig; 2005.

Rosenbaum SE. Improving the user experience of evidence. A design approach to evidence-informed health care. PhD thesis, Oslo College of Architecture and Design. December 2010.
Available at: http://independent.academia.edu/SarahRosenbaum/Papers/369198/Improving_the_user_experience_of_evidence._A_design_approach_to_evidence-informed_heath_care