I don't know if you would have any luck or not. What kind of material does it have to be? If you are using significant quantities, maybe you could have a custom extrusion done in aluminum or brass. These outfits will usually quote you a price if you'll take 3 or 4 tons of stock.
I also wonder if there is anyone doing custom powder metallurgy out there? I've seen some pretty nifty parts done that way. This one might be out of range.
Do you really need a full length spline or would a short bit on the ends work just as well? That rotary broaching video that I watched on here the other day showed how its done, and simply, too, on a regular lathe.