Sorry for question in old thread, but it is about the things I'm very interesting too.
So, this functionality is not implemented in bitcoin or any altcoin, am I right?
nLockTime is, mempool transaction replacement isn't.
I added a
warning to the contracts page explaining how mempool tx replacement is disabled and that implementors shouldn't assume it ever will be enabled. The page should be rewritten in the future to avoid reference to it.