Thats a shame (timing I mean). I am in the process of creating a USB box that will do just this (and will allow other controllers to be connected via USB), however as I am snowed under with work at the mo I really have not had the time to get my teeth into it.
I have a Megadrive - USB working prototype here, which I am going to use as a base for other adaptors, with DB9 joysticks high up on the list of controllers I am aiming to convert.
I don't have a ZipStick (although I could get one - or even convert your one?), but I could sort out an original Competition Pro which has been converted to USB, but it depends on how quick you want it? It would be either a DB9 to USB box or I put the USB circuitry and lead into the joystick itself and do away with the DB9 altogether.