I have a 200L that runs on 4 AA's. I wonder if the white emitter could be swapped out for a red emitter. Anyone ever heard of doing this?

Fashioning a red lens you could attach as needed might be easier and cheaper.

Doable. Buy a Luxeon III or a Cree XR-E in red mounted on a starboard, replace the stock LED and you're good to go.

A rider on BROL said he has used the red from a throw away pair of 3D glasses as a filter that worked quite nicely to change white to red for a rear light.

I couldn't find one on DX. Do you know who would sell one?

https://ledsupply.com/lxhl-md1d.php
https://ledsupply.com/creexrc-r.php
https://www.dealextreme.com/p/red-cre...-1-9-2-2v-1776

I'd get the Luxeon instead of Cree, the driver voltage might blow out the Crees.