Thank you for the clarifications. So I will update my avr_libc. Because I just started the project and do not know how much features will be necessary in the future I decided to go cooperative multitasking and not to use interrupts (power draw is not a concern that's why polling is ok). A small change to HardwareSerial seems inescapable for realization of non-blocking sends. Didn't want to modify the Arduino 1.0 in the first place...