Is it something the Joker would do? Certainly. I already see flashes of Jason in my mind, the way I found him, battered and bruised, the strikes falling any which way. Not in a pattern which would have taken him easily out of commission, but instead wherever physics would land the blows. And if the Joker could find some way to plant his consciousness into another person's body and take it over entirely, of course he would try.

But as far as I'm aware, there's no way for him to manage that without notice.

In that many years, however...

"What happened to Robin? Which Robin was it?"