I have an old 5810 that I painted Opalescent (pearl) and wanted LOTS of definition between the colours, so I used MANY coats of the pearl paint. Since the 5810 has higher keys than other handhelds, this didn't interfere at all.
On a 77xx there may be an issue, because it has keys that are just a bit higher than the plastic (not much sticking above).
On a 72xx however, it's got slightly higher keys than the 77xx so as long as you don't do 800 coats of really thick paint, you should be ok.
This is a quick pic of the 5810...tried to get it so you could see the green on one edge and the purple on the other (based on tilt from the camera and angle of the sun)....but the pic was done from a cell phone so it definitely does not do it justice.