This post will be difficult to understand as I cannot insert the xml to explain it with examples, however ...
I just had a look at a few files and this may be achievable (and if so absolutely awesome), using lightning attacks as a potential model.
If you look at the approach used for a lightning attack projectile it uses an XMB rather than a BRG and if you look at the XML script of one of the lightning formats they are defined as an XML type - lightning.
Now if you look at the XML of the Meteor GP you get a script defining a GP type - meteor.
Now, go and look at the Lightning GP and this uses a GP-type - lightning.
Now what if the GP-type-lightning is interchangeable with the projectile-type-lightning?
If it is, it would be possible to create a new projectile using the XML method used for lightning, but potentially using any GP-type (meteor, tempunit, earthquake, tornado).
If so you could cast a projectile that did anything a GP can do, all of the above would be possible. The only limitation I can see is that (as far as I can tell) you can only define one rechargeable attack action.
[This message has been edited by Stephen Caines (edited 02-06-2007 @ 05:03 AM).]