I was able to create a semi-kludgy work-around.

1. I created a new, non-private list in gossip.

2. i put one non-private npub in the list.

3. i put a bunch of other npubs into the list and toggled them each to private individually.

When i published this list from gossip, it was quickly available in Amethyst and the feed in amethyst does include all the private (encrypted) npubs from the original gossip list. I pulled down the kind 30000 event from my relay and confirmed that all the private npubs are in fact only existing in the content tag in an encrypted blob. The only clear-text ('p') npub is the one non-private contact i added in step 2.

This works but is a PITA.