... how about combining the frictional braking seal with the inertia weight?

My thinking: if the weight has a tapered front, and instead of hitting the end of the piston cavity it hits a tapered collet made of a slightly elastic material* which pushes out beneath an o-ring. The o-ring is slightly undersized so as to add no extra friction during the power stroke, and only comes into play when the inertial weight has run forward.
The taper would have to be steep enough so the weight doesn't get stuck inside the collet.

* The collet could alternatively be metal but in sections like a drill chuck.

Result: the inertial damping AND sudden frictional braking combined, without any extra friction.

I don't have the skills to test this; that's in the hardware domain and I'm more a software guy.