Paul Eggert wrote:
> this use of "+" does not conflict with input usages like
> "chmod +x foo".

It's because this use of '+' is easy to remember.
"chmod +x" means "add execution permissions".
"chmod -x" means "remove execution permissions".

You want a symbol for "assign exact permissions".
IMO the equals sign is a more well-known symbol for assignment than '@'.
So, how about a notation
    chmod =755
or
    chmod =rw-r--r--
?

Then there will not only be no conflict with MacOS X. It will also be
reasonably easy to remember for the users.

Bruno