Conformal Coating on LEDs
I thought of an LED that might work for your application. It features a ceramic package that makes it more environmentally robust and also conducts heat away from the LED chip more effectively than the usual clear epoxies they use.  You may be able to use a conformal coating with these, because of the ceramic package.  And because you said your application is indication, the optical properties of an additional, relatively thin coating, should not cause a problem.

The following is a table of ceramic package series from Cree that I think would work:



And here is a Digi-Key search that might help.

If you have a directional application, or an application where it's tough to remove the heat, it's probably better not to conformally coat your LEDs.