James Mitchell [1] is still with us and developed the first ARM instruction set, and went on to work at Acorn, Sun and Oracle doing interesting work.
Richard (Dick) Sweet was most recently at Adobe and just completed a 4000 mile Tesla trip around the America West.
I couldn't find any other info about William Maybury outside of his work at PARC. William (Bill?) are you out there in HN land?
Here is some additional info on Mesa. [2]
[1] https://en.wikipedia.org/wiki/James_G._Mitchell
[2] https://en.wikipedia.org/wiki/Mesa_(programming_language)