Web: consider adopting Bridgy classic's custom text mf2 property?

[snarfed]

Bridgy classic has a Keep Bridgy Publish dumb philosophy, which led it to avoid most features and settings and conversions around less common post types like bookmarks, plays, events, RSVPs, etc, especially when the destination doesn't support them.

Instead, Bridgy classic supports a custom e-bridgy-[SILO]-content mf2 that lets users provide their own text-first design content for destinations that don't support a post's type.

Should we support the same kind of thing here in Bridgy Fed?

Inspired by chat on #dev today.