There was no hiding involved or no restock for the event. I was at Barrelworks the past 3 weekends. Two weeks prior to Lil' Opal, they had the shelf stocked to the brim with Feral One's. The next week (a week prior to Lil' Opal's release), they still had them stocked on the shelf.
I can't speak for the weeks before, but given the fact that Barrelworks is only open 3 days out of the week and for limited hours, I'd imagine that the reason they stay on the shelf is the same reason Parabola, Sucaba and Velvet Merkin are chilling there. There's just a limited window of when to get them.
I'm sure if I stop by this weekend, or even the following, they'll still have Feral one's on the shelf.