I thought I'd share an interesting project with you. It's a master thesis project done at NTNU in 2010. It extends FreeRTOS with a tickless idle feature, using the RTC instead of systick, which enables it to go into EM2 in idle mode. In addition the idle loop is modified so that it will go into the lowest possible energy mode requested by each task. Each task can specify the lowest possible energy mode it requires by using the function vTaskCanGoToEM(int).
This port is made for the DVK, but the core should be useable for anyone using an EFM32. To try it out you must extract the file freertos6.zip into boards/EFM32_Gxxx_DK/examples. It requires the DVK_BSP and CMSIS packages from Energy Micro to properly compile.
I'm hoping that this can be used for a platform-independant implementation that can be included in future version of FreeRTOS.