Timber frame infill over cavity wall/steel lintels

[lookseehear]

Looking for some assistance with a detail:

 

Need to infill above this window on gable end (lintels are steel beams on inner and outer leaves). The builder doesn't want to use masonry because it won't tie into the steel and he thinks it's just going to end up unsafe. He suggested timber frame, but I guess it would need to be strapped down, which is again problematic over the window.

 

If we go with a timber frame, do you think straps either side of the steel would be sufficient? Timber frame just on the outer leaf? or on both?

 

Any alternative ideas?