Bashiok tweeted about this. "Rare mounts cannot be obtained from the Elder Charm roll. We want their drop chance to match all rare boss mounts."
But meanwhile on wowhead you can read many posts like this: "Also confirmed that Elder Charm of good Fortune has a chance of offering this mount as bonus loot. Sourced from a Game Master which I was speaking with." (this one is under the Elegon mount)
My friend asked a GM about it back in 2012 and I asked one some days ago. Both said that we CAN get mounts on bonus rolls. From raid and from world bosses too.
I wrote some tweet to Bashiok, but I didn't get an answer yet. However it would be great to know the correct answer since we can get 3 Elder Charms a week and there are 3 mounts (Galleon, Sha, Elegon) and if you would like to maximize the chance on the mount you have to burn all your charms on these bosses.
Since Blizzard misinforms us I would like to ask the community to report if they saw any of the mounts drop with Elder Charm roll and they are 100% sure about that.