Nope, with this current release its not possible. Its a very disappointing restriction of the software. I have talked to ESRI and its just not supported and there is no word when it will be. Seems like something that should have been there to me. I'm trying to do the same setup as you. Best answers were manage two portals, one inside (private) and one outside (public) the firewall, or use portal inside and ArcGIS Online for outside, (still managing two things). Or what we are doing just have the web adapter outside. Not what we want but only way to get where we need to be...
Chris
