I think that you're making it out to be slightly more difficult than it really is..., but still it would take some PHP programming knowledge.

I wouldn't bother writing a script to do all the processing, I'd write a script to break apart the forms contents into a way that each application's registration script can do the processing for you. Why reinvent the wheel when you can just use a bunch you have lying around waiting to be used?

It might be a little tricky but it would be easier than re-doing all the input validation.
All it really will take is figuring out what each individual script expects as input. I'd start by going to each registration page and going to 'view as source' and see what the form is outputting. Once you right that down you'll know exactly what each script expects in its $_POST or $HTTP_POST_VARS to function properly.
I'd think the easiest way would to be take the $_POST array you receive from your script and stick it in a $temp_array, then copy what you need back out to eht $_POST array and call the respective script's registration page (like coppermine's regsiter.php). That way you shouldn't even have to edit the applications themselves, you'll just be using their relatively well developed and stable code to your advantage and saving yourself a lot of coding.

That's just my thoughts.
Ciao,
Dhej