I managed to find a way to make the single post "extension" handled as an inline piece of data, instead of pushing it out of line. The solution ended up being elegant, not breaking the XSD in any way.
The approach works very nicely if the extension data is XML but also works for other kinds of data like free-formed text, JSON, etc. It just has to be something which won't invalidate the XML document (so things like inline binary won't work unless made into something like Base64 text rendering).
Today, I realized I could further generalized part of the user description, so I should do that before progressing with the full integration.
Still not sure if the new model should be enabled in 3.1 or just supported for reading in that version and properly introduced in 4.0.

I want to find the right way to incrementalize this,
Jeff.