xii-lat.tex would be found, if it were installed below
TDS:tex/plain/. But the TDS specification (or CTAN version without links) writes:
TeX macro files shall be stored in separate directories, segregated by
TeX format and package name (we use "format" in its traditional TeX
sense to mean a usefully
⟨format⟩ is a format name (examples:
The directory tree below
TDS:tex/ contains macro files like LaTeX classes, packages or plain macro files. It should be small, to avoid loading/parsing large hierarchies, when compiling a TeX document.
A better place is below
TDS:doc/. From the specification:
Most packages come with some form of documentation: user manuals,
example files, programming guides, etc. In addition, many independent
files not part of any macro or other package have been created to
describe various aspects of the TeX system. The TDS specifies that
these additional documentation files shall be stored in a structure
that parallels to some extent the fonts and tex directories, as
It is also the place for example files, and test files, source code for the documentation, ...
Therefore, the current place below
TDS:doc/plain looks correct for me.
A place for discussing is AFAIK the TeX live mailing list.