Can noun clauses be used as nouns? Yes. It's the only thing they can be used as. That's exactly why they're called noun clauses.
He will do what he might do contains the noun clause what he might do, but the sentence as a whole is difficult to assign any sensible meaning to. The people whose what they have in mind is very inappropriate were staring at my g
But the only thing that doesn't make it correct is the convention right? If I hyphenate it, it would be a noun and work, but if I don't then it doesn't go with the usage and it may be seen as wrong, right?