If you changed your result type to text, you could specify this result via an If or case function. But this will also remove the ability to use data formatting to apply a number format to your field, so you gain and you lose with that change. A calculation field with a text result could also be modified to apply any needed number format as well.
I'll have a look and switch between text/number.
A simpler option occurs to me that would allow you to keep your current field unchanged:
Put the - character on top of the field as layout text. Use the "blank when zero" data format. Use "hide object when" (New feature in FMP 13) to hide this character when the field is not zero.
Conditional formatting can make this text disappear in older versions.