OK, I got an answer from Disney. You are allowed to use their valet parking if there is room then go visit the theme park. Its not an issue with the self park because you can get to that the back way and park. Its just a bit of a walk because its across the street.
Here is what I was told:
Thank you for contacting Disney. Please hold while we route you to a Walt Disney World Cast Member who will help you with your question.
Welcome to Disney, you are now chatting with 'Mike', how may I help you today?
Me: We have a reservation for a few days at the Poly, followed by a 7 day cruise on the fantasy. After that we are returning to the park for one last day while we wait for our flight home.
Me: Can I park using the valet area at the Grand Floridan for that last day?
Mike: I would recommend parking your car at the Theme Park you are visiting. I believe you would be able to valet park your car at Disney Grand Floridian Resort & Spa. You can try parking there, but it is based on availability. They may recommend you park at the Theme Parks as well.
Me: Would I be allowed to park at GF if there is room available at the valet lot?
Mike: It will be one moment while I do some further research.
Mike: I did double check and you would be able to valet park your car at Disney's Grand Floridian Resort & Spa while you are at the Theme Parks. Please know that this is based on availability.