I don't know if it will work with your particular stem, but one of the great american-bike parts-makers, Wald, offers a stem/expander-wedge nut/bolt combination for so little $$$ you might try ordering one just to see if it fits ...
http://www.google.com/
The wedge is the bike-specific part, available nowhere else. I think you can get an M8 x 1.25 or whatnot hex-bolt (~ 135mm, correct length for my SR Royal Stem) for many applications (i.e. holding engines together), or get a local machinist to do it. It's also possible to get the same thing in an allen-bolt (10x10mm typically) configuration. It's even possible to get M8 x 1.25 x 135mm in titanium if you're a weight weenie ...
here are some examples that might work, it's probably feasible only if you replace both stem and wedge together at the same time.
http://www.bikepartsusa.com/
https://secure.actionbicycle.com/
The nitto wedges in the URL above are probably technomic because 230mm is almost 10 inches in length!
- Don Gillies
San Diego, CA, USA