Please diregard this if it is naive. I admit to not having much experience in this area.
But, if I can play devil's advocate for a moment...
I question the effectiveness of the letter writing route and wonder how helpful it will be. It seems that a letter like this might seem a bit pushy or taken in a way that suggests "Do you mind if we impose on your daily job requirements and give you more to do? After all, we are good customers." I know those aren't your feelings but letters can be impersonal and easily taken the wrong way.
It seems that this might be better handled in person, when you can build a bit more of a relationship with the person before asking for favors. I'm not saying that you don't have a right to those favors or that you are wrong to ask for them, especially if you are one of their better customers. I just think about how many people there are that use a "sense of entitlement" to become royal pain in the asses to every service person they come across, and even worse, then neglect to tip for this service. If the guy reading your letter just had a few encounters like this, they might not be so receptive to your letter.
What if it was more in the spirit of, "We just want to introduce ourselves. We are really looking forward to our visit, as we have always enjoyed our visits to your hotels. We would enjoy the opportunity to meet with you once we arrive so that we may learn more about the services available to us."
That seems less imposing.
Again, sorry for the hijack. Just wanted to share what my first impression was telling me.
