RobertL says:
"You dig out the brickwork through half the
thickness of the wall and insert the first U"??
It seems to me you are considering a double leaf wall...anyhow, how
are the bricks above kept from falling down? Using Strongboys?
(something I wasn't considering using in the first place). Maybe I'm
missing your point here or I'm being overcautious...basically I want
to play safe...racking some mortar sounds pretty safe to me.