Perhaps simplify your thought a fraction..
Adjust the cylinder head combustion chamber profile based on the timing requirements..have a lower compression at TDC, and increase the compression a few degrees after TDC...this would effectively perform the same function as your sprung piston?
CSRV vs. Poppet Valve - Coates International Ltd.
Using a cammed intake system similar to above, and taking changing the profile based on simple rotation. You could imagine taking this to the next phase, and implementing a Wankel style interface as the cam, instead of the twin cams used byu coates...and therefore simplifying the movement down even further..
Alternatively, electrically controlled "valves" could be utilised on a more conventional head...and if the valves opened / shut normally, but could be extended INTO the cylinder, to increase compression at the desired part of each scycle..(Making the valves a solid tube to take up volume..maybe twisting open / closed to vary between a solid tube and a fuel delivery / exit path)
Just some random thoughts to help with overcoming the requirement for 4 x truck springs being inside a 4" cylinder bore.. (Havent invented a Tardus as yet...sorry..)...