Thanks, again, Mick.
I'm leaning toward UserComments EXIF, rather than keywords, so I can extract the info as a discrete piece of data (for exporting text like the .csv list like in your example).
In fact, since I'm still using Media Pro, I will be exporting this type of info (camera model, film type, negative sleeve number, etc.) from my Media Pro custom fields to a .csv file that I'll then import into FileMaker Pro to make Avery stickers/labels. Media Pro is the ideal tool for that task. Of course, if I can figure out how to get that data into an XMP field, even better.
By the way, I am working with the client with test files to see if I can they can read Media Pro custom fields (they're supposedly embedded into the image), but I'm still waiting for the results. Oh, and I did ask the client what they use for a DAM, and the response was: "We don't use a single DAM; our systems are so complex it would take forever to explain." I didn't press.
I know PM doesn't have custom fields, but they'll probably be one of my feature requests for PM+. But I'm curious, if some of those EXIF fields are "not terribly obscure," why can't they be leveraged in PM or PM+ without the use of variables?