I am looking for the name of a play that was produced in the 1960s.
The story involved a woman, who with her son, became a fugitive 
(perhaps in a custody battle). Their refuge was the world of
international flight, beyond the wall of customs barriers. They flew
from country to country, never clearing customs to remain beyond the
reaches of the law.

I believe that the play was based on a true story. Any reference to
this story would be helpful as well.

I haven't been able to find your play, but I do have a suggestion:
many larger public libraries have, in their reference department, a
series of volumes called "Masterplots," now available both in print
and CD-ROM versions. The Masterplots drama reference enables you to
search through plays' synopses by keyword, which might help you to
locate a play whose plot description contains references to such words
as "mother," "fugitive," "flight," "custody," and such. You can also
search by title and by author, but obviously the synopsis search would
be more useful to you in this case.