Prefix Filter (Disabled Items)

The prefix filter appears to support disabled items:

[c] => stdClass Object
(
  [prefix] => C
  [items] => 1
  [link] => //localhost/example/glossary/prefix:c/
  [active] => 
  [disabled] => 
)

How would I utilize that, for instance, to complete the alphabet where certain letters don’t have content?

Thanks,
Dan