I keep this separate from variable size battlefields post.
Like you do for dwarven treasury and the like where you preload army position. Allow the map/mod creator to pre-define positions for obstacles. If he uses this option his obstacles are used, if not default random obstacles. ( Or even better a mix 
Define the obstacle you care about and let the rest be random).
It is mostly a feature request for future map editor and map format. [That won’t happen soon.]
Trivia: the possibility of manual defining obstacles placement on battlefield is already implemented for Duel Mode — special battle-only game, that I’m going to use to test quality of my upcoming battle AI module. Not actually available for “normal” game though.
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