Would it be much trouble to add the STM32F103 to the list of processors?
It also an ARM chip like the DUE
Can't say for sure. I've seen there is a Arduino compatibility library for STM not sure if that one is included. If it is compatible enough all that remains in modifying HAL.h/cpp to the special needs as number of timers or so may differ. But I think most can be used as it is.