The product we were working on when I asked this question was shelved, because of new priorities and serious doubts about whether we actually needed it.
Now I suddenly have to get it out, brush it off, and fix a couple of issues, so we can show it off at a user conference next week. Number one on the list of issues is this.
I've pretty much forgotten everything I knew about WPF, but I'm trying to figure it out, again.
I've dug through the UI tree using Snoop, and confirmed that IsEnabled=False for the controls in question. Then, just as an experiment, I hard-coded IsEnabled=True in the XAML declaration for the form. Snoop now indicates that IsEnabled=True for the controls - and they still can't be selected.
I vaguely remember having had to change the transparency on a layer that you draw over the top of the form, when the form was disabled, to keep people from interacting with it. Could something like that be in place, that would keep me from selecting the text?