Anyone aware of an open-source Help File Generator?  Needed for
authoring, not just reading / parsing.

The ITSS file format is used by Windows .chm help files, which
superseded their .hlp file format.  It is a complied help format. 
People go so far as to pubish magazines in .chm.

There is a wiki:  http://www/mshelpwiki.com  It lists A LOT of the
extant tools (they tend to be in the $199 - $599 price range.  I'm
looking for something costing $199 - $599 less than that) :-)