Just a thought.

The OP talks about a lintel across the inner skin. Does this cross the
cavity? I had something similar across a set of French Windows. The
inner skin of the cavity had a concrete lintel which was probably cast
in place. This was wide enough to cross the cavity and the height of
the soldier course. The soldier course was then built directly on top
of the steel Crittal windows. All fine until DG company removed the
window and the soldier course (but fortunately not the brickwork above
started to sag. Under these circumstances I wonder whether a piece of
angle iron that was screwed to the inner lintel at regular intervals
would provide sufficient support.

Andrew