I think, that this task can be done in a better way. You can use ussd-widget's parser option to start some script after the query was made. In that script you can wait for SMS and show balance on widget itself. The only issue is that parser invocation is sinchronous and would cause hildon-home to freese unil it runs. But, if you are intereted in this scenario, I can make is asinchronous.
What do you mean under short messages? This is very interesting news.