The important feature of the triangular key is that, when clamped, the wider flat face (the hypotenuse of the triangle) is wedged against the corresponding faces of both the column and head giving repeatable alignment. The design is very rigid compared to a bar set parallel to the column, where, apart from the flexibility of the rod, the bush on the rod would have to have some clearance to slide - although a pinch clamp would eliminate that source of error.
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As for needing to set the head at different positions at times, that is another matter, but not easy to solve as a modification to an existing machine. For example the whole column could rotate on its base, or a collar could be fitted for the alignment device, and the head clamped around that, free to rotate when needed.
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The option of multiple grooves in the column, or in the head, would call for a removable key, but that is no big problem. By having grooves in both, but different numbers, would allow a good range positions in a Vernier fashion, so a combination of five and six grooves would give 30 positions equally spaced around the column, but this is starting to get complicated.
