Why not just make every client have an encrypted indexer?
Wasabi is a light client, you need a full node to build an indexer.
(Unless I'm misunderstanding your post, the rest of the context confuses me)
Basically I'm assuming that the output of an indexer is just a list of silent payment addresses that it is aware of, along with some supplementary metadata.
The rest of the post is just a design idea on how to encrypt that list so that it can be saved on to light clients without compromising privacy.