After fully losing steam on this project, I'm picking it back up.
I got stuck because I realized how poorly I handled all persistent data. All posts (called them "passages" internally, which was stupid) exist within a single MongoDB collection. Rooms aren't an actual thing. A post without a parent ID is inferred to be a "Room". I'm not 100% sure what the structure should be, but I do know this ain't it.
Fixing this would mean breaking all stories, which would be a bummer.
I decided to stave off these data issues in order to actually make progress. Adding drawings to these stories was kind of the whole thing that made this idea feel special. So I'm going forward with getting drawings implemented. Starting quick and dirty, then I'll make improvements.
Last night I stripped down cadavre-exquis to re-familiarize myself with how the client and server talk to each other. This could still be stripped down into a much simpler demo, but this was enough for my purposes.
Right now I've modified the mush client to include the drawing canvas form, and added an endpoint to the mush server to accept image uploads. It's too jank to merge right now, but I hope to get it done enough to merge by tonight or tomorrow morning. :)