I dug a little deeper. I couldn't get your workaround to work, I looked at the Page Highlight color. In my layout, it was set to Auto. Auto is affected by the page Texture and Color. However, when set to black, all labels are black including the "On" button. The "On" button was "lit", all other buttons were not. This may be acceptable. If so, I can output the desired Midi set to "Send on only" directly from the Button Group buttons, eliminating the two additional sets of 32 buttons. On only is required by HW.
When Page Highlight is set to white in my layout, On buttons were black, off buttons were white. Thus Auto actually invoked white, since this is my original, unsatisfactory inverse button label colors.
I still would like to be able to select white button labels when on and black when off for extra clarity.