I don’t think that’s possible to do using activity pub, but if you have a matrix account you can add it in your settings. When viewing the profile of someone who has a matrix account set on the web ui, there will be a button that says “send encrypted message”. You should see it if you look at my profile page, for example.
there is also strong positive feedback loop, i guess… among the early adopters who are already here are people who deployed their own instance and found that only way to fix something is to do it themselves and commit the fix ;)