This feature has been implemented several times by various research groups and small companies over the last 1-2 decades. It is somewhat useful but it does not justify all the inconveniences of adding a tracker. It is also interesting to note that it is trivial to implement this feature for floor/ceiling-mounted C-arms (which already have encoders in the C-arm and patient table), this feature is not commonly used in these systems either.
If you already track the C-arm for some reason (for example, you track the patient and tools) and you can manage to keep the C-arm in the field of view then it may make sense to use it for helping with the positioning. You can also replace the clumsy high-accuracy optical tracker with inside-out tracking (MARG sensor for angle; camera or surface scanner looking at the patient and/or the ceiling for translation tracking) However, since positioning is still done using manually (most mobile C-arms are not motorized), it is not a huge improvement overall.
I keep hearing from companies the importance of dose reduction, but it is rarely a concern for patients and physicians I talked to are generally not concerned by the radiation. They often seem to accept higher exposure if it allows them to save time or make things more convenient.