"I am curious to see a C compiler make full use of that."

Well, on the Triscend E5, I have Keil C51 mapping bits of the Flash into XDATA, and paging bits of the off-chip battery backed RAM into other bits of XDATA, and mapping the on-chip RAM into still other bits of XDATA: all done in 'C' and without the aid of Keil's Banking support...
so I don't see why it should be a particular challenge...