See the "check boxes" layout found in the following demo file. They are not really check boxes. They are buttons with conditional formatting setup to mimic the functionality of a check box. If you are using FileMaker 12 or newer, use Open from FileMaker's File menu to open this file and convert it to the newer file format.
Note: in FileMaker 13, the new "Hide Object When" setting in the Inspector could be used in place of conditional formatting to control whether the "x" in the "check box" appears or disappears.
Thanks for the help!
The example still requires a portal and some scripting to manage the creation and deletion of the join records, but nothing too dirty. I had to alter my layout from horizontal to vertical list since there's no particularly graceful way to do a horizontal portal with varying-width text, but in the end, I'm pretty happy with the result. Using the same formula for the conditional formatted 'x' worked for the Hide Object When as well.
I appreciate the example; it got me to where I needed to be.