Hi John,
Happy anniversary to you and your wife! Fifty years of marriage is truly something to be celebrated, and how wonderful to be able to share it with your family at Walt Disney World. Though I do not believe that you can secure a spot in a parade, let me offer you some ideas to make your celebration special.
First, when you are making dining reservations or resort reservations, be sure to ask the Disney cast member to note on the reservation that you are celebrating your anniversary. Also, when you visit a Disney theme park, be sure that you and your wife pick up free "Happy anniversary" buttons and that the rest of your traveling party picks up free "I'm celebrating" buttons. You can add extra writing on these with a permanent marker. You could write "50th" on your buttons and have your family add their relation to you (such as "my grandparents" or "my aunt and uncle" and then "50th anniversary." By noting your celebration on your reservations and wearing the buttons, you are signaling that you have something special to celebrate. Although this does not guarantee any special treatment, you never know when a little extra pixie dust might come your way when you signal a celebration!
You also might consider commemorating this occasion with a portrait session from
Disney Fine Art Photography and Video. Some of my fellow Disney Parks Moms panelists have had portraits done with them, and find the results to be spectacular. The blog post that I linked show some example portraits and includes a link to their website and contact information.
I hope that these thoughts give you some ideas for your special celebration, John. Have a wonderful time planning your vacation, and please do not hesitate to ask if you have any further questions!