The magical custom buffer read-only

[Lennart Borgman (gmail)]

Custom mode buffers are, at least partly read-only. I want to discover 
if a custom buffer is read-only at point, but I can't. I have tested

   buffer-read-only
   (get-char-property (point) 'read-only)
   (get-text-property (point) 'read-only)

and

   (command-remapping 'self-insert (point))

I think the last one should work since in custom-mode-map there is a 
remap like this:

    (define-key map [remap self-insert-command] 'Custom-no-edit)

Is this a bug or?