Is there a generic way to access device specific functionality from within the MBED development stack or am I S.O.O.L and need to go to a lower level tool chain? (want to use the integrated op-amps of the STM32F303RE device)
You can just use the STM32 HAL functions like you'd normally do. Mbed OS wraps around these. Note that these functions are not guaranteed to be thread safe (unlike the Mbed HAL).