I think you would be pushing it .
The parade route goes around the Discovery Island. So if you were to stand close to the bridge going back to the Oasis (the wooded area you walk through after entering the park to get to all the stuff) You would be waiting about 10-12 minutes for the parade to get there and the parade is about 20 minutes long. So if it starts at 4 and passes you at 4:10-4:30 you would have to run to car 4:40, and the drive to MK is about 20 minutes (approx) and then park and take ferry or monorail (right? you have to park at TTC) you would be lucky to walk in MK at 5:30. Ok so you would be 15 minutes or so late would they still honor your PS. You could call and see if they have a later opening. Or ask them what if your a few minutes late. I'm sure people have been late before. (One reason is being on opposite side of park when PS time arrives..or being on a ride)
Find a map at Disney.com of AK and you should see the parade route. ANd then you'll understand what I mean.
I would not do it...just too much stress.LOL
You could call from the hotel the day before and see if anything opened up and then decide.
I hope that helps.
Lori