I think the biggest concern factor for this will be what the response time looks like when paired up against how much damage can be done in that time.
If the response time is long enough for significant damage to be done, it may be worthwhile to have some players stick around as a town guard during the window. It's a decision the settlements will have to make, but remember, you're not "safe" outside this window. The NPC guards can be overpowered. It's just harder.