Seems that win32/config.gc also needs amending. I've rewritten the 'd_inc_version_list', 'inc_version_list' and 'inc_version_list_init' entries in that file to:

d_inc_version_list='define'
inc_version_list='5.10.0'
inc_version_list_init='"5.10.0",0'
[download]

And, in config_H.gc, I have:

#define PERL_INC_VERSION_LIST "5.10.0",0
[download]

Perl now produces:

C:\_32>perl -V:d_inc_version_list
d_inc_version_list='define';

C:\_32>perl -V:inc_version_list
inc_version_list='5.10.0';

C:\_32>perl -V:inc_version_list_init
inc_version_list_init='"5.10.0",0';
[download]

That correlates well with what I get on linux, but still no /perl/site/5.10.0/lib in @INC.
I might take this up on p5p at some stage ... or I might just forget about it and settle for copying the entire contents of /perl/site/5.10.0/lib into /perl/site/5.10.1/lib.

Cheers,
Rob