Colin, two remarks on your solution:
1. The file extension .texi seems to be the standard one, but you should
also allow .texinfo, .tex and .txi [1]. An alternative solution would be
to scan the first row for “\input texinfo”.
2. My texinfo examples contain no whitespace at the beginning of input
rows and are thus not detected if they contain no “>>”. You should add
an additional space after the automatically inserted “>>” to make that
work.
Oliver
P.S. I have >260 files to check. A summary at the end of the file list
(FAIL count) would help me check whether there are errors at all.