[sorry if this message appears twice]

Hi Eliot,


>> 1. Have I missed some way to do this in FO or with any vendor-specific
>> extensions? (I've focused on XSL Formatter for this task simply because
>> they do inside/outside floats and XEP does not).


XEP indeed does have inside/outside floats.  For your problem, you may
establish block container that overlay both possible "offset" margins,
place float with float="outside" inside it, then insert a block that
re-establishes normal body margins, and place main text inside the
block.  Like this (assuming page margins are three inch wide):

      <block-container start-indent="-3in" end-indent="-3in">
        <float float="outside">
          <block>float-outside</block>
        </float>
        <block start-indent="3in" end-indent="3in">
          <block>the main text</block>
        </block>
      </block-container>

This will do what you want.

Cheers,
Sergey