Arrian (c135AD) wrote a treatise on how the Romans in Cappadocia could face off the marauding Alans (3Kn).
As is well known this involved amongst other things having the legions supported by artillery situated to their rear. In DBA of course this doesn't make much sense as artillery cannot fire overhead. This rule can be used to represent 'overhead' shootng in a rather simple (and dare I say abstract manner) - hopefully Arrian would have approved
The rule is as follows;
A "Solid" element of Auxilia or Blade in frontal close combat add +1 if fighting Elephants, Knights or Scythed Chariots, while supported by a single friendly element of Artillery that did not move that bound and is lined up in contact directly behind them or directly behind a friendly element of the same type in side edge and front corner-to- front corner contact with them.
Rationale: This enables Arrian's artillery dispositions against the Alans to make sense (vs Kn anyway - Elephants and Scythed chariots have been added by extrapolation - both being charging mounted as are Knights). Note this is really a static defensive measure for the foot as the artillery must remain stationary to give support.