This is an intriguing project, thanks for creating it! I'm guessing Journal is mainly a scratch-your-itch project intended to test the boundaries of what can be done with Matrix. In which case you probably have zero interest in supporting other protocols, and fair enough :) I just want to pop in and cheerlead for increasing the inter-operation between federated projects as much as humanly possible, by encouraging you to consider at some stage implementing a basic ActivityPub support (or a bridge to the AP-verse using something like BridgyFed). If you want to get really ambitious down the track, maybe you could become the first project other than Hubzilla to implement Zot.