Can a radial turbine be designed inside-out (sort of) with the centre fixed and the 'body' rotating - with the impeller veins curving in from the outer wall? It would run in a outer bearing shell.

The larger the radius of the machine, the more the rpm (for power take-off) would drop. This avoids the inefficient 'dragging' the gas flow would suffer in a normal RGT caused by 'tightening' the vein geometry.

Outer rotation velocity being a fraction of an equivalent centre shaft machine the GT has built-in reduction gearing.