There's no way to have a G_OPTION_ARG_CALLBACK option that doesn't consume
an argument, meaning you can't have a no-arg option with custom behavior.
There are a few things that this functionality would be useful for:

  - Toggle args, where passing it once turns it on and passing it again
    turns it off

  - Args that count the number of times they were passed, like
    "tcpdump -vvv"

  - In garbage-collected languages, you generally can't take the address of
    a variable, making most of the arg types awkward to use (you have to
    allocate temporary memory, copy the initial value of the variable in,
    pass that to GOption, and then copy the final value out at the end).
    In Gtk# at least, it would be easier for us to tell glib that all of the
    entries are ARG_CALLBACK entries, and use reflection to get the values
    into the right variables from the callback. But right now this won't
    work for boolean args.

This could be implemented either by having a new G_OPTION_ARG type or by
having a G_OPTION_FLAG value. (Since G_OPTION_FLAG_REVERSE is specific to
ARG_NONE arguments, having flag values specific to ARG_CALLBACK arguments
doesn't seem like a big problem.) If it was going to be done as a flag, it
would also be nice to have a flag saying that the callback expects to
receive a filename, as opposed to a plain string, so goption can do the
right encoding-munging in that case too.