I think if you are starting need elements like this then you are veering away from a straight forward low risk contract that the short contracts are intended for. Having said that, I can see that it may be straight forward work and you just want it complete in phases, so perfectly feasible that it would work and you don't need to revert to the full ECC.
All you would need to do is add a line into the contract data to state what the sectional completion(s) is(are). I suggest you would also need to look at the wording of X5 and incorporate the elements you need as a Z clause into the contract. You may also need to beef up your definition of Completion to make sure it is clear what needs to be achieved in order to achieve (sectional) Completion. Lastly you will need to make it clear whether the delay damages are the same for the Completion Date and Sectional Completion(s) and if not have separate lines for both.