If your final destination is Houston, then you wouldn't have any access to lounges on arrival given you need to clear US customs and immigration before being dumped out of the airport (and you need a valid same day boarding pass departing the airport to clear security). However, if you are connecting through Houston, you would also be able to access the excellent Polaris business lounge at Houston upon showing your boarding pass for the SYD > IAH flight. Otherwise, if you are later departing from Houston you could certainly try the Centurion lounge, again provided you are holding an AmEx Platinum card.
Hope that helps.
-RooFlyer88