If one has a grinder, making a key slot on a steerer might not be rocket science - for those with great eyes and hands. I'd use a narrow fiberglass metal-cutting blade (<$10), and build a bit of a jig to keep things parallel and prevent cutting too deep. And practice with a bunch of discarded forks, first. The "key" is that these slots aren't precise, unlike the keyways on motors.
I've done this once or twice to open up seat binder assemblies, freehand (w/o jig). At least once, it came out looking a bit crooked, but that was once when it didn't matter at all.
Of course, my wife's lovely diamond-blade tile cutting saw would be perfect. It has a nice rolling table and pretty good precision on depth of cut. But, I'm not sure that it is worth the risks associated with potential drawdown of "marital capital" and a fairly expensive blade. :-)
harvey sachs
mcLean va.