I am trying to find an authority for the rules by which Western languages are composed into lines, in particular, the rules for where line breaks are allowed.

What do you mean by authority, hopefully not something on the order of
ISO?

Do you mean something like this:
http://www.bartleby.com/141/strunk.html#8

Although perhaps a philologically oriented rule, as in the tradition of
line breakage was established....?

E.
--
W. Eliot Kimber
ISOGEN International, LLC
eliot@xxxxxxxxxx
www.isogen.com