I spent a lot of time building complex systems using various models of PDP-8, PDP-15 (18 bit!), and PDP-10 (36 bit). I don't remember ever resorting to packing ASCII characters in the fashion described. Instead, characters were almost immediately transliterated into SIXBIT code, which allowed for packing 2 characters in a 12 bit word, 3 in 18, and 6 in 36 bits. One lost control characters, lower case letters, and a few other symbols, but given the limited capability of the output devices at the time, this was hardly an issue. Most of the assemblers, compilers, and interpreters we used at the time limited themselves to using characters from the SIXBIT set.
The PDP-11 was the first computer I used ASCII on to any great extent. The endian issue was already being dealt with by ARPAnet protocols in 1975, when I was involved in interfacing the first PDP-11s to that network.
If you really want to know how open source software came about, ask Richard Stallman about EMACS and James Gosling...