It was said in the Patchnotes, and here on MMO-Champion before that "/use Mountname" would no longer work till a next update and that "/userandom" would also not work and might have been implemented at a later date.
Just checked on Live, -it does- work again.
You can change the names into whatever mounts you use. The first part up onto the ";" are the flying mounts, separated by "," if summed up normally. After the ";" is what will be used if your macro variable "Flyable" is not met. These are also split by ","#Showtooltip
/userandom [flyable] Swift Green Gryphon; Stormpike Battle Charger, Reains of the Swift Stormsaber, Swift Yellow Mechanostrider
Hope it is useful for some of you.
Defias Brotherhood EU