I don't doubt that your changes were not made gratuitiously.
Please grant me the courtesy of believing that neither were mine.
And, sorry, but I just don't want to take the time to delve into
building lilypond and figuring out from scratch where the issue is in
your sources. I looked at your Texinfo files under Documentation/ but
nothing jumped out. I can easily believe the issue is related to use of
@macro.
I did just verify that the small input file below using macros to make
index entries involving backslashes worked as expected with the current
texinfo.tex, so far as I can see.
If you, or anyone there, can send a (not necessarily minimal) Texinfo
file that demonstrates the problem, I'd be happy to look into it.
Karl
\input texinfo
@setfilename mac-idx.info
@macro refcommand{WHAT}
@cindex \WHAT\
@cindex @code{\\\WHAT\}
@code{\WHAT\}
@end macro
The @refcommand{paper} block.
@node Unified index
@appendix Unified index
@printindex cp
@bye
