@amoroso, the serial-term isn’t a comint buffer (it’s more primitive), so the comint commands won’t do anything. I tried adding delays to the comint code, but I haven’t figured out how to get that working yet.
Per the “; (semicolon)” entry in the Ulisp Language Reference,
A comment line turns off echo, so that by starting a long listing with a comment
it can safely be pasted in to the Arduino IDE's **Serial Monitor** without
overflowing the serial buffer. Echo is turned back on automatically after
a one second delay if there is no activity on the serial input.
So you could try putting a ; line above your code and including it in the region to be sent. But I just tried it and I had weird results in the serial-term, I think because the (insert l) function actually inserts the text into the serial-term buffer, so Emacs may be trying to display it itself every time it receives characters from the ulisp side (as it will for every definition, where ulisp will print out the symbol defined and a prompt). So we may have to live with the echo for now.