I wonder if the same functionality could be or should be provided for standard modules too? What about the general case of CPAN?

It would be very cool if there were some way to extract such information, but to stay realistic, that won't work until Perl has function/method signatures.

Currently much documentation actually duplicates the argument passing code in some way

=item C<foo($bar, $count)>

Frobs the object by doing C<$bar> at least C<$count> times

=cut

sub foo {
   my ($self, $bar, $count) = @_;
}
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This duplication is mostly done in a non-standard way, and as thus really hard to automatically distract.

I don't think that you'll have overwhelming success with convincing CPAN authors on a standard documentation format. The only way to win is to enable function and method signatures. By hacking them into bleadperl, or by helping Perl 6 development.