Not sure on exactly how you get round keeping your helmet compliant and painting it but clearly it can be done. Have always wanted a custom painted rally style helmet.

Be carefull of putting stickers on the actual fibreglass part of the helmet as I have heard that this can also make the helmet not comply. Need to pull out the CAMS manual and check. But would only really affect you for serious CAMS events. Wouldn't have any dramas at the drags and other smaller track days.