I am just thinking out loud here.
As I remember, It is one of the most often requested features to do with units in the past. So others have already thought of the benefit.
I once was asked if I could create a units extension to have part of a model in one unit and an older part in ancient units. (There are many archaic unit systems that were used to build ancient buildings that need to be modeled because of restoration work, or they need to interface with new additions in modern units.)
So imagine a project in China, where a new addition needs to be made to an ancient building. The ancient part could be in a group. Imagine being able to set the ancient group to have it’s own “local” units set to one of the archaic local chinese unit systems that would be in effect whenever working within the group.
Currently users have to work in a separate file in say meters, and then scale it or a component instance inserted into a large model file down or up to the “special unit” using some scale factor.
[FYI. The requester wasn’t willing to fund an extension, he wanted it to be a SketchUp native feature.]
For example the model generally may be done in cm, but flooring (tiles, etc.) might be calculated and purchased in another square unit. So modelers often have requested the ability to “tag” certain surfaces to display in the unit which they’ll use to buy the covering (tiles, flooring, paneling, siding, etc.)
In the U.S. the building modeling units are “ft-in” but the surface area units are usually decimal square feet.
But it is has been the metric users that have said their area and volume units differ the most from length units.
Another example is concrete volume which is ordered in cubic yards in the U.S.
Still another example is roofing and siding materials. In the U.S. they are calculated and ordered in squares. (1 square is 100 sq. feet.) Material rolls are 3 feet wide and 1 square total area (33-1/3 feet long.) Singles are typical in 1/3 square packages.
Some modelers could get by with a global setting in Model Info > Units, others might like more control.
To be fare, it might be done in a plugin, … the unit and value kept in attribute dictionary.
Windows has an established style for controls that are set as the default but can be changed. It is not an insurmountable problem.
I cannot see how a “Local Unit” dropdown control in the Entity Info panel is “difficult to design” or in any way not “intuitive.”
Simon’s idea is that the right portion of the Area and Volume boxes are dropdown unit selector controls.
I don’t think it’s all that bad of an idea myself, and it serves to condense the info panel. I believe it would be absolutely intuitive if a unit choice control follows a value, that all users would understand that changing the unit in the control will affect the displayed value that precedes it. (They would be in reality separate side-by-side controls.)
Or perhaps the new “Special Attributes” could serve as the repository and display place for special units.
The “special units” would display down in the special attribute list and be easier used in LayOut documents where the information is more useful (or Reports.)