so I made it work on my own fork. but not sure how well it fits within the framework.
I'm using same pattern as for MUTABLE_CONTENT_KEY/CONTENT_AVAILABLE_KEY to pass custom live activity hash that gets put into the aps payload sent. Very simple patch - can send if there is interest?

It's a bit not intuitive how to use though still - user has to pick which token to send start/update/end. I'm not sure what kind of api change would need to happen in rpush to make them be more properly represented.