Hey Cutts,
Took some time to solve your issue looking and testing the shitty formatting properties but got fedup and changed strategy. 3 quick steps and you got both full flexibility in terms of formatting, as well as it works like a charm. Here is what I came up with...
1) Database side: Add a column with Absolute version of your column with true/calculated numbers.

2) Use an update statement to ensure the values of AbsoluteNumber and place it in your appropriate SQL triggers in indoogrid (under Onload, after record update or whatevah)
Update NNTEST
Set AbsoluteNumber = ABS(Number) FROM NNTEST
3) Choose 1- Configurable for the AbsoluteNumber column and do the following quick config:

(Sorry forgot a row more with Column (AbsoluteNumber), blank pre- and suffix and a "Number =0" condition so that you dont get blanks whenever your number is 0 ;-))
The "Number" column itself, make it invisible ;-)
I think its a quite flexible solution since it can also be used for currency etc. No "fixed" formatting. But yes an extra column for the jungle ;)
I used Numeric with two decimals and heres what I got (sorry for the strange font ;P)

Have FUN,
Dave