[kh99]

One thing you might do is this: create a new plugin using hook location pmdata_start and this code:

if (is_member_of($registry->userinfo, 5))
{
    $this->overridequota = true;
}

but I should mention that this would allow a supermoderator to send PMs in other cases where they would otherwise be denied, not just in the case where the recipient has turned off PMs. (I think they would have the same PM sending abilities as admins).