Hi Diane,
For your requirement, commonly implemented solution is installation of a non-central Advanced Adapter Engine (ncAAE) together with dedicated Web Dispatcher or any other load balancer for it, both placed in DMZ. It isn't necessary to install AEX in DMZ since you already have PO in your landscape and you only need runtime engine for messages processing in DMZ - so ncAAE is normally sufficient.
Regards,
Vadim