Are you producing the report?

This one is one of the most stupid requirements I ever seen.

Zero, as a numeric thing, cannot be negative. Some times it used for showing temperature in weather to show that it's just low enough for water to freeze.

As soon as you add - to 0 it's stop to be numeric and starts to be a formatted string.

I would only do it in UI (eg. reporting tool).

If this is used for some sort of data extract, it's still will way better to show it in two separate columns. One is numeric value (0) and another one, CHAR(1), sign.

Than you can pass whatever you like.