Sounds like an interesting thing to add to your games. Have you looked at The Perfect Captain’s Hoplomachia rules? It’s been a while since last I read them, but they had a system for pre-battle sacrifice that might give you some inspiration.
Another consideration is that ‘magic’ like religion would have been a lived experience and not differentiated from any other aspect of daily life, so perhaps it would be better to incorporate its effects directly into the game systems instead of having it as a separate entity. I’m not really sure how you would do this. Perhaps a system like Dark Age Infantry Slog where you spend time building up the morale and fighting power of your troops to the point where you think you are sufficiently ahead to launch the attack might work.
Whatever you decide upon, I look forward to reading about it and how it works in your games.