1) Do you really need a RTOS ?. In a workloop environment it is dead easy to start every task with "if you have nothing to do, get out."

2) If you have an unused interrupt, you can set that at a lower priority than all other interrupts (easier if you use a 4 level priority drivative) and by toggling the activating flag create a "task" that has a priority between actual interrupts and the main.

Have fun,

Erik