As Marty already stated - on the controller side you could limit the value - but that would get back the issue of semi-transparent layers...

So best solution here is again a black image @ some percent intensity on top of all other layers.

It could be an interesting option to limit single colours of the RGB system as alot of LED fixtures overpower blue (and sometimes red) compared to the display on a computer monitor (green LEDs output less light at the same power...)
So a limitation of single colour (as part of the fixturetypes.xml) would be a nice option... - Sure you can colour correct all of your footage for optimized playback on your LEDs - but it is quite time-consuming...