that's the what i thought for first... it should be not so complicate to bitbang an rs232 frame.. divide the clock and the baudrate to have the delay between two subsequent bits, send the start bit, transform the char in byte and send the 0 or the 1, send the stop bit.
That's exactly how Tiny Debug Serial works.
BrainBooster:
But with this method in my tests, if i send "aaaa" i receive sometimes "aaaq" or sometimes the right string, or randomly "qqaq", i guess because of the internal oscillator and his low precision.
The untuned internal oscillator is probably why serial communications does not work.
do you think TinyTune may help?
It has helped me. If the temperature and voltage is fairly stable I've been able to tune the internal oscillator to ±0.5%. Which is certainly good enough for serial communications to work.
and why when i use the arduinoTiny core i'm not able to do it via the tinydebug serial at 1mhz?
It worked when I tested it. 
Try tuning the processor.