Of course
The data is sent as follows:
http://www.vatsim.net/fp/?2=BAW444
Results in this because the 2 in the URL refers to field 2.
http://www.vatsim.net/fp/?2=BAW123&3=A320
Results in this because the 2 in the URL refers to field 2 and the 3 refers to field 3.
So, in your schedule briefing, you can make it:
http://www.vatsim.net/fp/?2=<?php echo $schedule->code.''.$schedule->flightnum;?>&3=<?php echo $schedule->aircraft;?>
Remember to try and fill in as many fields as possible for the pilots, so that they have less filling out to do.
Then, make this a link for your pilots:
<a href="http://www.vatsim.net/fp/?2=<?php echo $schedule->code.''.$schedule->flightnum;?>&3=<?php echo $schedule->aircraft;?>">VATSIM Prefile</a>
Then click the link in your schedule briefing and it will bring up the relevant data Mine currently does 10 of the 16 fields. You can add even more if you have the VATSIM ID custom field, as you can then get their ID too.