Yikes, drilling in hard steel is one thing, but tapping is another.
You could consider chrome plated shafting (usually for hydraulic cylinder applications), and hope that the chrome will stand up to a bit of use, as it is quite hard and low friction, too.
However, normal chrome plated shafting is not hard under the plate, but is just a C1045 grade steel bar. If anything strikes it, it will dent.
If you want, you can also buy induction-hardened, chrome plated, which would have the maximum life, and would be dent resistent, too. This gets you right back into the tapping problem. You would need to clearance drill at least 1/8" deep to get through the hard case, then you can drill and tap in the softer interior just like normal.
The straightness of 3/4" shafting is not something that you can rely much on, so you really need to have a straight rail to fasten it to. In that case, you might as well get something a little higher class to start with, like those fancy aluminum extrusions, or a larger diameter round bar.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)