You can summon a dragon (I think it was from a dragon shout)... meaning that dragon will be friendly to you.
So maybe you can ride that summoned dragon as well?
Just a thought.
I'm severely doubting that it is "friendly" towards you. The power of the Dragonborn is more than normal men, and probably allows you (with the shout) to invoke your will upon a weak dragon (much as you can control the minds of men with magic if you're strong enough). I'm sure the dragon is not happy about the situation, but has no choice in the matter. After all, a lot of dragons perceive the mortal races as lesser beings not worthy of living.
That said some dragons have helped the Empire in the past and it's possible you do get friendly with one. That seems somewhat obvious and cliche, unless Bethesda are very careful about how that plays out. I certainly don't want it to feel like a "pet". It would have to be done in a way where the player realises the power of the dragon, and is actually humbled by it's friendship.