Is the deployer supposed to be able to add extenders to an operating mechanical piston?
I tried to use a deployer to feed extenders to a working piston. The issue is that despite getting a fresh extender by the deployer the piston does not use it.
If this is not a bug could this feature be eventually implemented?

Thanks for the report.
Pistons only re-attach blocks when stopped and then re-activated. I'm sure there is a way to automate the extension, but the general idea is not to dynamically mess with the moved structures' composition during its movement.