I thought about this some more and come up with another "simple"
approach that didn't require me understanding too much code, but does -
I think - address your concerns.

I've changed the heuristic to avoid any throttling on PF_LOCAL_THROTTLE
task if:
 - the global dirty count is below the global free-run threshold.  The
   code did this already.
 - (or) the per-wb dirty count is below the per-wb free-run threshold.
   This is the change.

This means that:
 - in a steady stated, all bdis will be throttled based on their (steady
   state) throughput, which is equally appropriate for PF_LOCAL_THROTTLE
   tasks.
 - a PF_LOCAL_THROTTLE task will never be *completely* blocked by dirty
   pages queued for other devices.  This means no deadlock, and that is
   the primary purpose of PF_LOCAL_THROTTLE.
 - when writes through the PF_LOCAL_THROTTLE task start up from idle -
   when there is no current throughput estimate - the PF_LOCAL_THROTTLE
   can be expected to get a fair share of the available memory, just as
   much as any other writer.  This was the possible problem with
   treating PF_LOCAL_THROTTLE just like BDI_CAP_STRICTLIMIT.

So I think this is a good solution.  Thoughts?
Patches follow - I've address the comment formatting issue.

Thanks,
NeilBrown