Okay, I got it sort of working thanks to @cybereality's functions! It now renders as a decal on the screen and has normal map support, but something with the view angle/direction isn't totally working:

I think it's probably just something small that is slightly off, as it seems mostly right sometimes. I needed the dFdx
and dFxy
functions to work with Vector3's as well, so I made a function for it as well, but it's possible that maybe that's where the issue is with the normal map? I'm not totally sure but I'll try to take another look at it soon to see if I can figure it out.
I also had to remove the animation part of the shader though, as in GLES2 it was causing the shader to render everything as completely white. The code is still there, but it's commented out. I also noticed that in GLES2 the glow environment effect doesn't work very well because of of the lack of HDR.
I've attached the project with the progress I've made currently.