When Greek used Qoppa, it used it with back vowels generally. I don't
know enough about the use of Q in Etruscan to address it one way or the
other, but I mistrust any claim of "spelling conventions" in the
pre-classical period. Ancient peoples seem to have adjusted their
spellings pretty easily, whether by changing spelling to reflect a
limited change in sound, as witness the "-OS" -> "-US" shift in the 2nd
declension, or by altering the value of a letter to reflect a global
change, as in the bifurcation of "C".
--
John W Kennedy
"Those in the seat of power oft forget their failings and seek only the
obeisance of others! Thus is bad government born! Hold in your heart
that you and the people are one, human beings all, and good government
shall arise of its own accord! Such is the path of virtue!"
-- Kazuo Koike. "Lone Wolf and Cub: Thirteen Strings" (tr. Dana Lewis)