I have a feeling you're not going to get sufficient coverage if you simply 'rubber band' an umbrella to the top of your flash; it'd probably only hit the bottom half of the umbrella.
If you're going to use it for running and gunning, have you considered alternatives such as Westcott's Mini Apollo or Lumiquest's Softbox III? They're probably not going to be as effective as an umbrella, but they're more non-light stand friendly.
Otherwise, I'd assume that you would have time to prepare/make adjustments, in which case I'd suggest you just man up and get a half decent light stand (they're not ridiculously expensive). They're not exactly that hard to move around and adjust, especially if it's just a speedlight and umbrella.
If you have someone helping you and you have a lot of cash to burn (which I doubt, since you're trying to save money), you could invest in Lastolite's Ezybox, which has the option of an extending handle that an assistant can use to position the softbox for you.