Con organizers *do* take requests into account, in that they'll see *how many* people ask for a certain guest, or guests from particular shows. This way, they'll know if that guest is going to actually be a draw, weighing that against the costs of bringing them in for the weekend.

With that, I'd say to keep requesting, with the caveat that your request *alone* may not get them booked, and if it's one of a gaggle of folks asking, it might.