Thank you for the comment!
The SIGNAL usage in HardwareSerial.cpp is very likely from when it was originally written (2006). I suspect no one ever changed it simply because there wasn't much motivation. Why change code that's working?
In regards to the Arduino Tiny core, the first goal was to make a "highly compatible" core. To accomplish this goal, I tried to change as few things as possible; the code in HardwareSerial.cpp is almost identical to the 0021 version.
Oh, and I forgot to mention in the previous post...
Caveats
- In my testing, HardwareSerial did not work at baud rates above 9600 when the processor was running at 1 MHz. This could be a problem with HardwareSerial, the fact that the oscillator was not tuned, or a bit of both.