The capnp structure itself could be a hashtable, and doesn't need to be (entirely) loaded into RAM.

I'm saying you create a capnp list, and then you store elements into the list at position according to their hash -- i.e. how you'd build a hashtable, but the capnp list itself is the backing array.

You would have to do this at write time, of course, and make sure the hashing is consistent between runs.