I was wondering if it would be possible to configure cache policy in the page table entry (short descriptor format) in such a way that the cache is used only if the data already exists in the cache? A kind of write-through / "no-allocate" policy?
Not on the Cortex-A9. The "short descriptor" format (as used by the Cortex-A9) has an implied read-allocate on the cacheable types. It was the "long descriptor" format that added controls to separately control read and write allocate.
Note, that the allocation attribute is a "hint". The processor might ignore you.