I don't believe it can happen in the TT rules (I have them, but never played).
Also, the in-game manual in the PC game mentions the risk of Commanders falling in close combat, but I couldn't find a reference to missiles.
I have seen commanders falling in the PC game, but always in close combat I believe. Maybe even only in melee and not impact, but not sure about that.
In the TT game maybe the chance for a commander to die from missiles was seen as too big when rolling 6-sided dice, so it wasn't implemented (however triple 1's on 3d6 might do the trick imho).
In the PC game it could be a percentage chance based on the number of casualties caused. For example 1000 strong unit suffering 8 casualties from missiles would have a 0,8 % chance to lose the commander, or maybe 0,4 % if you consider he might be better armoured, guarded, only lightly wounded and still able to issue orders etc...
However, I feel there should always be a small chance of such a thing happening. Not too big, but not zero chance either. Think of king Harold at Hastings, for example. If it isn't already, it should get implemented before that scenario comes out!



