If I understand, you want to add a little diffusion to the beam, like an orange peel reflector would, but you want to achieve this at the LED. I think, by acid etching, you hope to change the shape of the surface without adding any new material, and in that way, avoid reducing output. If so, neat idea.
I can't tell you about acid etching. But, I have added diffuser material in 2 ways, one on purpose, and one by accident.
On purpose - Reflector: I simply sprayed a tiny bit of automotive clear coat on a reflector, from 2-3 inches away so it would go on wet instead of dry spray, but not cover the whole surface. Just splatter. This helped smooth the beam. I'm sure it reduced output a little. It required practice to spray a small enough amount. Line up the can first, and just tap the spray button as little as you can while still spitting paint. I had also tried 'dusting' a reflector with clear coat from further away. Maybe 18". This made it MUCH easier to get the right amount, but it gave a more even, dry spray instead of a bumpy clear spray in only some areas. I thought the dry spray would waste more light.
By accident - LED Dome: I was using very thin CA glue to attach a 3W LED to an aluminum bar for a test. I put too much glue down, and it stayed wet for quite a while. Some vapors from the CA appear to have condensed on the LED lens and caused a frosty coating. You might play with this concept if you have a few LEDs to destroy in testing. CA is quite clear.
I'm guessing this might hurt output more than the clear coat method, because it is a frosty looking coating. But I really have no idea.
Maybe make a little fog chamber from a pill bottle or similar, with a hole in the top to expose only the lens.
