You can catch the bus from BCV to MK. Then either take the boat launch to the Poly or the monorail. On the day that you are going to the luau, if you have already been to one of the parks that day.
You might want to catch the monorail from Epcot to MK.
Depending on what show you are attending that night. You may want to check the time MK closes that evening. Perhaps someone else can help you out on this, but I believe the bus stops running 90 min. after the parks close.