In the GTK+ changes from bug 70541, it's clear that
pango_layout_set_base_direction() would be nice to have,
but there's no way to pass that information into
pango_itemize().

Also, it would be nice to be able to have HTML style
<span dir="rtl">blah</span> in GMarkup, rather than
having to do do bidi formatting with &#x200N; escapes

The solution here, seems to be to add some sort of 
PangoAttribute that allowed setting Bidi direction.
Since you want to allow nesting, you can't just have
an attributes with values of RTL/LTR. A scheme that
perhaps would work is to have a integer-valued property
along the lines of the resolved bidi-levels.

So,

  001112222
  abcdefghi

Would result in

   ab<LRE>cde<RLE>fghi<PDF><PDF>

being passed to fribidi. It's relatively easy to implement code
that takes a UTF-8 string and attribute list, inserts control 
codes, calls pango_log2vis_get_embedding_levels(), and then strip
the control codes out of the result, 

A point neding consideration in the above scheme is perhaps what
(if anything) to do about overrides.