I'd overcome the problem, now there is a possibility to file custom PIREPS for custom routes. I used the form max suggested and also disabled the schedule check by removing the following strings from core/modules/PIREPS/PIREPS.php:
if(!SchedulesData::GetScheduleByFlight($this->post->code, $this->post->flightnum))
{
Template::Set('message', 'The flight code and number you entered is not a valid route!');
Template::Show('core_error.tpl');
return false;
}
The only problem is that if the airport is not in the airport list yet you have to add it manually, no matter before or after you accept the PIREP. But that doesn't seem to be a big trouble, really.