I am fitting 9" joists, which means i'd need 6.75" noggins. However, a 6"
noggin would almost be resting on the tops of the truss joists, which
means I cant use to-the-spec noggins, and the metal straps are basically
out of the question...

Just use wooden herringbone, as discussed elsewhere. Cut up battenning
material for it, particularly easy if you have a compound mitre saw. The
6.75" requirement is just the distance between the top and bottom mounting,
as it were, it doesn't all have to be wood.

It is much easier to fit than noggings, as any slight error in length is
accommodated by adjusting the angle of fitment.

Christian.