ChibiOS/RT is compact and efficient RTOS that can be used in most of microcontrollers like AVR, ARM, MSP430. And now it seems that it can be done with soft-processor called Nios II. As yo umay know Nios II is a standard software processors widely used inside FPGA chips. Michael from yagarto.de wrote a two part tutorial on how to set up a Nios II processor on Altera DE1 board with Cyclone II (EP2C20F484C7 FPGA). First part of tutorial mainly covers how to program Nios II processor in to FPGA, while second focuses on configuring and building ChibiOS for soft-processor.
ChibiOS port for Nios II is already written – it just needs to be attached to set of RTOS. If you have Altera DE1 board laying around – definitely give a try.