If you really want to do this in the way you describe then you need yet another plugin like the one being discussed for read_count. There are two reasons:
- Using composite columns to display ratings is hard. It is especially hard to display 1/2 star ratings. CC will never display them as anything except numbers.
- If you use a visual rating column (custom or otherwise) then the rating must be recomputed during each calibre session where the read_count could change.
A plugin could look at the values in read_count and set a custom rating column to the "right" value.
I am not interested in writing such a plugin. It has zero value for me, but I would be required to spend time to maintain it. At this point in my life I am busy shedding responsibilities, not adding new ones.