Just a little follow-up, I updated one of my clients yesterday with the "portal as button" solution and found that Filemaker for Windows handles this slightly differently than the Macintosh client.
On Windows, the contrast color (Black in this case) still appears, but only for the top portal row. All the additional portal rows handle the button as expected, and actually a little better than the Macintosh, they don't show the "flash" of triggering a button.
It's a curious inconsistency, but ... whatever.