Your real options are to take the bus or boat to DD and then a bus or boat to POR. You could also do that by going to any Theme Park.
POR is close enough to walk from OKW, but it is discouraged because you'd need to cross a very busy roadway with no sidewalks or crossways. There is an unmarked construction trail thru the woods, but it is now pretty overgrown and is inhabited by local creatures often inhospitable to human tresspass. (Ask me how I know - )
I'd suggest taking a bus or boat and allow 45 minutes to an hour for the trip although it may take less time.
I'm not 100% sure either, but I just checked allearsnet site and it states 11 pm for POR as well. Scroll down to "what can you tell me about transportation" : http://www.allearsnet.com/acc/faq_dxl.htm
...To Downtown Disney (including the Marketplace, Pleasure Island, and the West Side) Simply take the bus marked Downtown Disney. There is also a boat to Downtown Disney (hours are 4 PM to 11 PM).
We just stayed at Port Orleans Riverside in March. We took the boat back from Downtown Disney one night at 11pm and the driver stated that it was the last boat.