Well I think it's impossible but what do I know..
At least it's the hardest thing to do because AI doesn't like to fire anything but real targets. Even a GL won't do for them.
I found this in the FAQ
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QUESTION:Anyhow I was wondering if there is a way to create an explosion in midair?
ANSWER: If you want to get ordinance exploding in mid air, try camcreateing two of the ordinance at exactly the same position. Eg:
_big_bang = _ordnance camCreate [x1,y1,z1]
_big_bang = _ordnance camCreate [x1,y1,z1]
Due to the two shells being in the same place, they will explode. Looks good as flak, especially at night.
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A neat trick to use but I don't have anything concrete on your question, sorry.