The stem riser slides over the steerer so that's why... Or perhaps I missed something?
I will ask them if they can put surface bosses, then you can drill. The bosses they use protrude through the tube wall.
I was thinking of surface bosses on the outside of the riser. Still will have to remove the bolts and bottle cage to take the steerer off, but I don't see myself doing that very often.
I would like a 59cm top tube equivalent (measured center to center)