Hi Bruce,
You're right that statement fields cannot be modified with formulas - only the spec codes available to you may be printed.
If it is simply change positive amounts to negative, you could use a text label to print a "-" in front of the amount fields. Of course this would not change negative amounts to positive...
Could the customer optional fields in Receivables 8.0 be of any use? These can be printed on statement specs. There are two amount fields which you can enter any value - perhaps useful if there are only a couple of negative values to enter, but not very useful if it's the details you want the sign changed...