Thanks, Henrique!
The formula does work, but requires the block be inserted in Model space, unfortunately.
In Civil 3D, the scale bar (and north arrow) are 'linked' to a Viewport, from which they extract the necessary scale (or direction) information. I can do similar with a Field, however referencing the Viewport object only yields the Standard Scale as string, such as "1\" = 40'".
Using a Field that references a Formula, which itself references a Field using [0.5\1.0\2.0] / <CANNOSCALEVALUE> works perfect... But again, requires the Scale Bar be in Model space (Paper space == 1:1), which now means also accounting for the 'dynamic' scaling of the Scale Bar block itself, unless I want to introduce another manual task. Model space can also have a different Annotation Scale, than the Paper space Viewport, which could present an issue (I purposefully make them the same, but they're not 'linked').
... Grumble, mumble, grrr... Need some coffee.