> Someone pointed out the arm26 port, which may have already
> achieved this.

The ARM 26-bit mode is entirely mundane.  It's conceptually
similar to the 68000 with 24-bit addressing, except that the high
six bits are used for flags rather than being ignored.  Come to
think about it, that's more similar to an IBM 360.  Anyhow, it's
nothing that poses any substantial challenge for running C code
or Unix.