As pointed out, there are way to many variables to what, when, and where damage would occur from falling drop tanks.
The coading effort to make this realistic would be, IMHO, a monumental programming task.
As Guppy pointed out ... in the scheme of things ... there are far bigger fish to fry in AH coad before HT might even consider adding a damage factor to falling drop tanks.
It would be cool, but the Return on Investment is extremly low.