I've just been playing with an STM32F4DISCOVERY board modified to run the real time clock. I've used ChibiOS to give the RPi access to the board over a USB virtual com port. The RPi can get and set the date.
With a little extra work, commands can be implemented which will allow the other peripherals on the microcontroller to be used - examples are available in the ChibiOS source.
More details at http://www.wheeze.plus.com/RPi_and_STM32F4DISCOVERY/