Using miles or a certificate will not give you bonus miles.
I find it hard to believe you can't sneak in an 800 mile o/w trip - which can be done within half a day. Look in the Mileage Run Forum or look for flights
from AUS-IAH-XXX and back. If you do that, any flight will get you over.
If you really can't fly, buy the miles. It's about 13 cents a mile with Elite Maximizer.