It's possible on UNIX too:

$ hg init b
$ cd b
$ touch b
$ chmod -r b
$ hg ci -Am0
adding b
trouble committing b!
abort: Permission denied: /home/mpm/b/b

If I've said I want file b in my commit, commit should obviously fail if b can't
be read.

Adding interactive bits to usually non-interactive commands is trouble too: it
will cause scripts to unexpectedly hang forever.

on windows it is possible to allow a user to list a directory, and at the same 
time disallow opening a file in this directory for reading.

it "hg adds" such a file without trouble, but bails out when doing a "hg 
commit". it would be much nicer to print a warning / question which allows to 
continue the commit anyway.

leaving non-readable files out of a commit would be a possibility also, but i 
guess it slows it down unecessarily.