Well first I'm building it (and will fly it for a while) totally stock, then will consider whether it's practical to modify it or just build a new one with alternate mechanism.
I'm not convinced a piston per se is necessary. The ejection charge should push just fine on the nose cone, just need to implement the ability to have the two halves of the rocket slide apart about 1/4-1/2" without fully separating. An unglued coupler and a piece of internal Kevlar to allow movement but not separation, would do it, but exactly how to attach the Kevlar internally at the correct length is something I've not figured out yet. Doing it externally would be very easy, albeit unsightly and a bit draggy.
I was pondering some sort of 3D-printed thing, but haven't worked out the details yet.