USB to RF bridge: Kettler Ergorace bike
If you're willing to butcher your device, you can find traces that go to the CP1202 and connect one of the many bluetooth-to-serial adapters. Make sure to find one which is pre-programmed accordingly and doesn't rely on the host device that it's connected to initializing it with AT commands every time, else you will be stuck. I don't remember which is suitable for this purpose, but i recall i have seen some.

As an alternative to Pi in the same vein, but smaller power and form-factor, but without display output (hey, you're not making a PC), you may want to consider Arietta G25 and its sibling, CoreTech WiFiG25.

If you were aiming for the cheapest possible hardware, it might be PIC32MX210, some of them have USB Host and cost &lt;4$. To that, you connect some serial Bluetooth. I won't be able to tell you how easy or hard it will be to program, because you might have to do the USB CDC support yourself. In the same vein, other microcontrollers would be in the PIC24 series and AT32UC3B.