Radio Group Multiple Selection
How about if you hold the control key down while clicking on buttons in a radio group you can have more than one button active at the same time, or if more than one already is active you can disable them one by one?
The Radio group feature uses an existing implementation from JUCE directly and this would involve hacking around in their codebase.
I'd say this is a scenario where you would have to roll a custom solution using script panels (all pieces required for this should be there)
Ok, I'll give it a go :)