Thanks for your reply fjakkarin, after almost two months I’d pretty much given up on receiving any response.
Although the internal Home Assistant notifications work really well, I’m somewhat discouraged from using them since learning they’re built on Firebase Cloud Messaging.
Meanwhile, Signal is my family’s primary messenger, so having Home Assistant’s notifications go through Signal has some real appeal, albeit there’s reduced functionality (e.g. it appears there’s no ability to define message-specific notification sounds).
I did open a ticket with Signal to ask about the API at the same time as my original post above, but to date have not received any answer from them.