RMT in XI had no control over the actual chocobos. It was something you would rent from an NPC in certian cities and villages, and even at the tele-points. To obtain a chocobo, you would need to be a certian level, then you would have to do a side quest in Jeuno (the central city) in order to obtain access to riding them. The only thing I can think of that made obtaining access to ride them difficult was maybe..and I say maybe because I don't know, is when it was first implemented, the RMT could have bought all the side quest items (I think it was gyshal greens?), and then mark them up extremely high on the AH. That is the only way I can think of that RMT would have had any control over getting access. Hope this helps. Also, if you are curious about the chocbo quest, you should be able to find the actual quest instructions here on ZAM somewhere in the FFXI section.
"I've never watched a nuclear explosion myself. That's a couple of degrees of stupid above my limit"- Old Man Harris