Leaving this here as a possible solution for other people who find they encounter a similar problem.
The problem was that the documentation on Qukeys in the wiki was outdated.
kaleidoscope::Qukey(0, 1, 7, Key_LeftAlt), //Backspace/alt
ought to be
kaleidoscope::plugin::Qukey(0, KeyAddr(1, 7), Key_LeftAlt), //Backspace/alt