Well...I actually have some experience in this area.
DH and I were chosen to be Grand Marshals in the Share a Dream Come True parade on our honeymoon. What did we do to get it...uhh...we were really lucky! We were talking to a CM in a suit who was handing out guide maps first thing after the park opened...I was trading a pin with him, and DH got chatty with him. I wanted to hurry up, because all these people were streaming past and I wanted to get to Space Mountain (DH had never been to MK before). Fortunately for me, I didn't rush them, because in a few minutes the suit asked us if we were going to be staying in the park for the parade...I knew something was up, so I said yes, and he asked if we wanted to be in it. To say I was excited would be an understatement....I started shaking! He had to ask me if I was okay - LOL!
So, we got to ride in a car that Walt himself used to drive at
Disneyland, and they gave us a certificate to prove we really were grand marshals. Oh, and we got personalized mouse ears to wear in the parade. Other than that, we don't actually get anything, other than the experience of a lifetime.
I'm sworn to secrecy with the CM's name who chose us, but I can tell you that it's pretty much random. Be nice to those you interact with, look like you're having fun, and everything else is luck. For us, I think it was because we were on our honeymoon.
It won't hurt to ask, but I've also been told that usually for being grand marshalls they won't take anyone who actually asks about it, because they want it to be random. (We asked at City Hall last year at
MNSSHP, because it was my nephew's first trip and we thought it would be an amazing thing for my niece and nephew...they told us they'd already picked for the night, though.)
I think what you propose saying above, though, is great! Go for it! It absolutely can't hurt anything, and you never know what magic Disney might work!