Why do you need to turn it straight back? Wait until 16.00 and then run return service. Whilst this would be close to afternoon peak I am not sure how long does it take the XPT to get out the Brisbane commuter area.
You can probably say 20 minutes. The peak service and loading is well and truly under way by 16.20.
The reason a set isn't left parked up all day at Brisbane (or indeed anywhere else) is simply that there isn't enough rolling stock available to provide the service if that were to occur.
You would either have to find another set which doesn't exist or cancel one of the existing north coast workings altogether.
XPT diagrams stretch the fleet pretty thinly over the ground. That's one reason why a delayed Melbourne is all too often turned back from Albury. If it doesn't get back to Sydney more or less on time it stuffs the roster for the next working as well.
If there were more rolling stock I'd go along with those who suggest using an Xploder to Dubbo but then the XPT set released is a short formation and may be unsuited to use elsewhere.