It's come a long way from the dark days before Travelnet when you were completely at the mercy of the gate agent and their mood. At least now you can SEE that you were denied a seat your seniority and the rules state should have been granted.

I really don't have any advice as to who will give you anything more than lip service but I would lose the word "complain" from your message. I would state something more along the lines of "I wish to bring this violation of our company policy to your attention..." "Complain" is a gate agent word attached to anyone who disagrees with them and makes you sound frivolous from the start. It seems like you have a legit issue and ought approach it as such. Good luck and let us know what haps.