Serial with DMA and 9 Bit Multiprocessor mode

I’m working on a communication with RS485 bus and want to use the Multiprocessor mode that the STM32 MCUs provides.
Its working already roughly, I could use SerialBase as a base, fortunately it supports already chars >8 Bit.
DMA seems to be only prepared, not implented (for STM targets). Are there some resources for DMA based serial? Is there a roadmad for DMA usage in Mbed?