Can ICU be used for a database's default sort order?

I tried to arrange $subject via

create database icu encoding 'utf8' lc_ctype "en-US-x-icu" lc_collate "en-US-x-icu" template template0;

and got only

ERROR: invalid locale name: "en-US-x-icu"

which is unsurprising after looking into the code, because createdb()
checks those parameters with check_locale() which only knows about
libc-defined locale names.

Is there some way I'm missing, or is this just a not-done-yet feature?

regards, tom lane