Would it be possible to print a baudrate error during the build?
It was fun, since I wanted to be sure it would work on systems with only the minimal Arduino-IDE compile tools (no "developer tools" on mac, no C compiler on windows.) I ended up using the avr-gcc preprocessor to generate a shell script :-) (ok, so that might have been going overboard.) make pro8
make atmega168 AVR_FREQ=8000000L LED_START_FLASHES=3
BAUD RATE CHECK: Desired: 115200, Real: 111111, UBRRL = 8, Error=3.5%
avr-gcc -g -Wall -Os -fno-inline-small-functions -fno-split-wide-types -mshort-calls -mmcu=atmega168 -DF_CPU=8000000L -DBAUD_RATE=115200 -DLED_START_FLASHES=3 -c -o optiboot.o optiboot.c
optiboot.c:254:2: warning: #warning BAUD_RATE error greater than 2%