If you don't mind getting your hands dirty you can modify core and change PostTools.parse
me -> 😨
I was thinking to extract the topic id from data.url and pass it back to the plugin backend to store, and then limit my plugin to one instance per topic for now. But I think that would be even worst, and when 0.6.0 arrives I would have to fix it anyways.
I am a bit scared to touch the core stuff, but I may give your suggestion a try since it is less hacky, and better long term solution.
for the case your plugin gets activated after user-creations you'd have to use the hook action:plugin.activate (take care to compare the id you'll get with your plugins id since this fires for each plugin).
The following is the way I'd go within, maybe there is a more simple (or better performance) way:
Get somehow all user-ids (personally I don't know how, maybe this would help) and use the function setUserField of the user-module to set each users field-value.