I think welding a nut onto your hull is overthinking it a bit.
I'd probably be inclined to treat the light as any other below-the-waterline through hull. Pick your location, drill your main and bolt holes, bed the puppy with a liberal amount good sealant, and through bolt it. Since your hull is steel, lock washers and hex nuts should be sufficient.
Thanks, I think I'm gonna 'just do it' like you said and not worry too much about it.
As for electrolysis, I'll just isolate it with some nylon spacers. But worse case it would be the $100 aluminum light that goes, not the hull (right!?) In any case, it'll be in a place where I can inspect the backside easily.
As for power/heat, these are LED and really quite nice little lights. I'm also planning on using some as spreader lights.
As for the pimpin light... can't help it! I am a redneck and grew up in a trailer park. I have about a billion projects for my boat and so where did I start? I bought some Blue LED speaker grills of course!
Yes, I realize how ridiculous that is.
I did do some actual work on it though:
inline images seem to be broken, and there should not be a space and link in text inside a img or code tag...