According to maemo wiki you can flash on the fly but I don't think you can flash n900 without holding down the U key and making sure the USB icon is shown and stays on.

What you can try is cold flashing n900 which is:
- Making sure n900 is turned off and the USB cable is not connected to the device.
- Take the battery out for at least 30 seconds.
- Plug in the USB cable without the battery in, slide the keyboard out. Obviously the other end of the USB cable needs to be plugged into the computer with flasher in standby mode.
- Make sure your finger is on the U key, insert the battery and then as soon as Nokia screen comes up with USB logo quickly press and hold the U key.

Cold booting isn't easy but eventually you'll get it.

As for the --usb-device switch. I personally think that only works under linux. Though in windows you can probably enumerate the same set of ID (theoretically) via checking the device ID as it appears under the device manager. It will be in one of those hardware strings on the last tab iirc.