These all pose the same problems, correct. At the
moment, I'm very unsure about
how to accommodate these at all. Maybe we can have them as "custom units",
which
are fixed for a given property, and can not be converted.
I think the proposal to use wikidata items for the units (that is both
base and derived SI as well as Imperial units/US customary units) is
most sensible.
Let people use the units they need. Then write software that picks up
the units that people use (after verifying common and correct use) by
means of their Wikidata item ID. With successive versions of Wikidata,
pick up more and more of these and make them available for conversion.
This way Wikidata will become what is needed.
I fear the discussion presently is about anticipating the needs of the
next years and not allowing any data into wikidata that have not been
foreseen.
There may be a way that Wikidata can have enough artifical
intelligence to predict which unit prefixes are usable in common
topics versus scientific topics, which units shall be used. Where
Megaton is used (TNT of atomic bombs) and where "10^x ton" are
preferred (shipping). And that the base unit for weight is kilogram,
but for gold in a certain value range ounce may be preferred and
gemstones and pearls in carat
(
http://en.wikipedia.org/wiki/Carat_(unit) ).
But I believe forcing Wikidata to solve that problem first and
ignoring the wisdom of the users is the wrong path.
Modelling Wikidata on the feet versus meter and Fahrenheit versus
Celsius problem, where US citizens have a different personal
preference is misleading. The issue is much more complex.
Gregor