i3dm:
any other options?
perhaps using another pro mini with hw serial in a stack mounting?
but as far as i can tell, not even hw serial is available with 9 bits without modifications that are beyond my level?
instead of a pro mini, you may want to try a Leonardo Pro Micro which has an additional Hw serial (Serial1) and dimensions close to a pro mini.
and it's not as complicated as you think it is! most of the post you already looked at (and wrote there) has instructions on what to do.... you just need to follow them
I've also attached a 9 bit serial library which you may use if you wish... its the same one as the one in my 9 bit serial post which you saw EXCEPT you do not need to replace any files. Just put the folder in the 'library' folder of your arduino IDE program and try out the example.
Good Luck!
HardwareSerial9bit.zip (13.6 KB)