I know with HTML you can have <<BO.attr @ ###,###;(###,###)>>
but you want to do this with the actual attribute format. Not sure if you can do that.
Of course, for the query, you could have a plain text field that is updated when the actual field changes and is formatted properly, but that won't solve the problem of the user tabbing out of the field. I am not sure if you would get stuck in an endless loop, but could you have the following javascript in your form render script:
var cq = parser.getField("customerQty");
$("#" + cq.getId ()).focusout (function () {
if (cq.getValue() != cq.originalValue && cq.getValue() < 0)
cq.setValue('(' + cq.getValue() + ')');
});