Hello Dora-9 ...
Yes ... I can see that without a seat type parachute the problem is equally acute. I'm surprised it's not a mandatory Flight Safety MOD requirement on Civilian Chipmunks. If it were me I'd have the MOD done.
Best ...
Coff.
PS. The original Chipmunk 4 Point Seat Harness ...
Sorry ... but I can't find a pic of the 5 Point Seat Harness at the moment.