I'm following an ancient bit of advice by Father Ramon, submitting
selective migration requests through the command line.

It seems to choke when I submit more than somewhere between 150 and 300

Any idea what the cause is (I ruled out malformed XDS, when I was able
to submit two separate 150-query docs, constructed from a 300-query doc
that failed.)

Configurable? I need to migrate 24,000 "select" nodes... beats a
resynch of 641,000. Ideas welcome. :-)