I hope you've got an MIA-HNL booked before that to match up with it.

You have to fly the HNL-MIA first. It's then the MIA-TLS that you can reschedule for later if you wish.

If don't fly MIA-HNL somehow to use the HNL-MIA, your MIA-TLS will be cancelled and you'll lose everything. Very bad use of miles in that case. I bet you would no likey that!

Just to clarify another points:

This trick may save you a one-way award ticket. It doesn't generally save you a round trip award ticket. You still have to "buy" a one-way ticket the other way. (But since AA prices round-trip award tickets at the same price as two one-way tickets, there's no "pricing" downside to making AA roundtrip award reservations as two one-ways instead of one roundtrip, and it may add signficant flexibility.)