I got my choco whistle and able to call it out almost 2 hrs ago or so. Unfortunately mine is yellow though, and when i called it out, it was just like riding a regular one but slower, and only stayed out for 15 mins total >.<
I tried in Xarcabard and glaciers too, you can't, but it's pretty neat how you can use olduum ring and call it out. I did that. I also dug with it a bit outside Windy, and breeding my choco with my friend's one in Upper Jeuno.
You get a Choco card F and M from each, one character have both cards, then you pay for a dating plan, trade all three to that npc, and wait around for the egg. I'm betting it's one day or until JP midnight.