Well, if you NEED the power of a RPi, than you probably have needs above what FreeRTOS is designed for. As I said, multiple cores are one big issue for FreeRTOS, as it is fundamentally based on a single core design, which means it can ignore a lot of the problems that come up with multiple cores. It also means you likely want Process Isolation, where you have multiple processes in your system and one process is very limited in what it can do to another. FreeRTOS does not support that.
As to examples of boards that can run FreeRTOS (maybe better to say work well with it, since as I mentioned, I suspect that you CAN run FreeRTOS on the RPi, it may just need a bit of peripheral porting work, and that FreeRTOS will run on just a single core) would be things like the Arduino modules, or the development kits from most microprocessors of this class of single core processors.