Dear Walter,
With hollow core precast RC slabs (the type used for floors in buildings), they create hollow cylindrical ( or rectangular) voids that run horizontally, and are centred about the neutral axis of the slab/beam. The general idea is to remove concrete that is doing very little structurally ie the concrete that is close to the neutral axis.
I would guess that the use of steel hollow sections as the formwork for creating these voids would be a waste of steel because the steel itself would be close to the neutral axis. It might be better to use cardboard rather than steel.
IMVVHO
Best wishes
Martin