Yeah, full circle back to my first post in the thread.
The more I think about this the more I think highlighting is not the right approach for this plugin. The problem is that I am trying to highlight rows which need attention by the user. If they use the likes of the bulk metadata edit dialog to edit only a subset of the rows, they are going to lose their highlighted rows and have to run the plugin action again to see the rest. Plus either I select all the invalid rows for them which gives them no individual review or I only select the first which makes it a pain for them to select the rest.
The custom column approach has some challenges of its own but perhaps is a bit more flexible/robust in this situation.
EDIT: While I was typing you guys have come up with what sounds a nice solution to the "challenges" I had in mind... very cool.