I think the main USP of Coolsnaz's methodology is its blistering speed, it's easier to crank the speed right up to whatever the highest speed the lowest common denominator (the SC/MP in this case) can cope with when using synchronous (data + clock) serial data as he is.
If using asynchronous serial, assuming you would want to adhere to a standard baud rate you also run into the 4.00Mhz / 4.43MHz problem, where the timing values would need to be adjusted accordingly for one clock frequency or the other unless you were using a very low baud rate in which case you might get away with setting the MK14 baud rate at a half way value between the ideal values for 4.00Mhz and 4.43MHz.
There is already an existing USB Serial Hex to MK14 programming interface here -
https://www.vintage-radio.net/forum/...d.php?t=175912
- needs a bit more hardware of course, but on the other hand requires no modification to the OS PROMs and the host machine with the code on it only needs to have a native terminal program capable of sending text to a USB serial port.
This will shortly be joined by DavidMS's functionally similar Pi Pico based version which also has an onboard display / UI so that 'popular' software can be stored on the interface and sent from there to the MK14 without the need for a host PC.
https://www.vintage-radio.net/forum/...d.php?t=197465