I think there is some way of telling CPAN testers to emit n/a for those operating systems

I think it's just a matter of beginning the Makefile.PL code with something like:

exit 0 if $^O=~/MSWin/;
[download]

That should put an NA into the matrix for any module being built on Windows - no report gets sent to the author.

But I think the exit value is important, so it would be best to verify (eg on the cpan-testers mailing list) that 0 is, in fact, the correct value to specify.

Cheers,
Rob