What about a two ply visor? 2 way reflective material in the rear and clear visor outward with LEDs framing around between the visors. Maybe material covering the LEDs to hide the individual lights. Not sure if that would give a full face light affect. I also don't know if you wouldn't end up blinding yourself.
there is something called skull skin and there are motor cycle helmet visors too. I ve never seen these in real, but I found it while looking for a green visor for my free. they say you can see though it perfectly. I didn't found an all white version but it must be possible. maybe this in connection with a two ply visor like Mara Jade´s Father said could work. but I don't know If you could find an all white one
skull skin visor
yes, exactly. I think I would they it this way: one clear lens with these one way white vinyl on the front and a mirror foil at the back, so that just the holes won't be black but silver. so you have a mostly white reflective lens. than a second thick lens made out of fever glass or acrylic glass and white LED s around it. because everything s reflecting, you should nd blind yourself but should be able to look trough it. I think that would be the easiest way. Another idea I ve got is to do the whole visor not to see through. so just take a milky white acrylic glass in that shape and put a layer of LED s behind it. than you d have to build a kind of a periscope maybe to the small place were the visor and the forehead meets. I can't see it clearly at the drawing but it seems to be there is a little outstanding structure. If you d do this to see trough and put a mirrored visor there, none could ever recognize it. there your periscope would have to go to. so, you would see trough this little stripe OVER the visor. I s a little bit more work and there s of course not so much space in the helmet, but the effect would be great.
However, if you actually want it lit, your options are pretty much "fully lit with little-to-no visibility" or "lit from the perimeter which will give more of a edge-glow effect, and only slightly limit your visibility. The fully illuminated effect would probably require an LED field (like Guy-Manuel's Daft Punk helmet) with some kind of diffusion screen placed over it. However, that leaves you unable to see with the helmet on, so it may just have to be a photo op type thing and you carry it around the rest of the time.
Lighting from the perimeter can work, but first you should try something. Take a flash, hold it up to your face with the emitter pointing across (perpendicular to) your field of vision, and turn it on. Take note of the sensation and effect that it has on your eyes, and then imagine that same effect, but from every angle around your face. Your mileage may vary, but my experience is that it is unpleasant. That said, with this option you can leave the helmet on all the time, but just turn the lights on and off when you need them.
I don't want to discourage you, just to give you some realistic expectations. The costume itself is awesome and I would love to see a real-life version of it!