i don't think mercury vapor phosphors would work with LEDs. Just like MV and fluorescent lamp phosphors aren't interchangable since the UV waves they emmit are different. Philips has done what you said with their houshold lamp. it has blue LEDs with yellow plastic panels. I think removing the cover would work like you said. I've been screaming this to the fixture manufacturers since the start of the LED street light take over: ADD A FRIGGIN DROP LENS TO THE STREET LIGHTS!!! Drop lens HID street lights distribute light better than FCO HID street lights so drop lens LED lights will distribute the light better than FCO LED street lights. Or even just some sort of diffuser to cut back on the awful glare that LEDs have when looked at from underneath.
I now have an LED street light and i noticed something else that a refractor/diffuser could correct: the multi-shadow effect. Since there are multiple LEDs, there are multiple light sources and when there's multiple light sources, there's multiple shaows. I'll try to get a pic...