Previous format was unusable.  New format is ranges of hex values.
To choose space character and Latin capital letters for example:

$ fc-pattern ':charset=20 41-5a'
Pattern has 1 elts (size 16)
	charset:
	0000: 00000000 00000001 07fffffe 00000000 00000000 00000000 00000000 00000000
(s)