In the Dartmouth collection of XFNC / XCMD code also some similar thing is available: Look for "SerialHandler". However, some system software extension might need to be removed. As far as I remember it is called "Serial (Port) Arbitrator" which causes SerialHandler not to work. Unfortunately I have no running system for comparison at the moment, as my appropriately configured PB 180s refuse to boot :-(
Probably a simple terminal application like ZTerm (scriptable!) would be sufficient for experimental transmission of a few bytes. You might transcode dual to decimal numbers with regard to the bit order to input and send the byte as an ASCII number. I successfully used a simple HyperCard stack with SerialHandler XFCN in the stack ressources to set the parameters of a stepper motor controller. If you have difficulties to set up communiation, check port parameters, cable and correct transmission. You may loop back the serial port to the second serial port using a printer cable to directly check for proper operation. For debugging of serial comunication I used to use an additional computer in the middle, running a little program to display any input from one port and send it without modification through the other port and vice versa.
Probably a simple terminal application like ZTerm (scriptable!) would be sufficient for experimental transmission of a few bytes. You might transcode dual to decimal numbers with regard to the bit order to input and send the byte as an ASCII number. I successfully used a simple HyperCard stack with SerialHandler XFCN in the stack ressources to set the parameters of a stepper motor controller. If you have difficulties to set up communiation, check port parameters, cable and correct transmission. You may loop back the serial port to the second serial port using a printer cable to directly check for proper operation. For debugging of serial comunication I used to use an additional computer in the middle, running a little program to display any input from one port and send it without modification through the other port and vice versa.