Ivan beat me to it. We had this discussion elsewhere recently and a few ideas were suggested. The PCB supports ISO but you'd need a new plate. You can use an ANSI enter but you'd need to rig up a blank above it somehow.
The only other thing I can think of is perhaps after adding the ANSI enter you could cludge an MX switch into the hole where the \ should go, supporting it somehow. Once that's done, it might be possible to wiremod it in as the \ switch? I've not done any wiremodding so I might well be wrong on that one. I think there's space though, as there's a large hole in the PCB underneath for the pins/wires etc.