Shadow Maps - prevent updates

Hi. Even with intense googling, I can't find an answer.

My problem is that I have a scene almost static. However, I have a lot of lights (can be up to 100) which, even if they don't move, can be turned on and off. Thus, Unity "lightmapping" is quite problematic.

My idea was to use "standard" shadowmapping, BUT ONLY PRECOMPUTED, a.k.a. built the first time only (i am assuming here that the bigger cost of shadow mapping is rendering shadow maps, please, tell me if I am wrong).

So, is there a way to disable shadow maps "update" but still using it in surface shaders?

BR, BilboX