While I agree that LuaJIT is great piece of work, the linked page says Mike is not accepting sponsorships at this time and there doesn't seem to be any way to fund him.
The best way to contribute to LuaJIT seems to be to learn its internals and make contributions, or to fund somebody else to do so. Mike encouraged this when he gave a talk on LuaJIT in Zurich last year. His own commercial ambitions seem to be already satisfied at the moment.