@Silvia, @lmayencourt, We have created and implementation of the rmw layer using Micro XRCE-DDS, a middleware specific for microcontrollers. Regarding the cross-compiling topic here I think you may want to check how we cross-compile part of the ROS 2 stack into an STM32, using an RTOS like NuttX.
The RTOS and cross-compilation interesting parts are in this repository (Following NuttX way):
https://github.com/microROS/apps/tree/feature/microROS_stm32-e407
It is a WIP project so any feedback, idea or question will be helpful.