And quick glance at femtoOS made me think it's indeed what i'm looking for.
Well, I see no contradiction between OS and the lib. I'd have divided all of robot's logic into tasks (in fact I already did) and would use freeRTOS or FemtoOS for managing tasks and webbotlib as a framework for easy access to different hardware (servos, radars, LCD, etc). So far, in my mind, they could be integrated with ease.
My biggest problem is lack of hardware to experiment with, as all the MCU, servos and sensors stuff I ordered will be shipped not sooner then june. (Yes, I live far from places where all the robotics fun is going on
). Meanwhile I'll keep up experimenting with software.