OK, so it turns out I'm an idiot and I didn't know how the stock pumpkin bombs worked. Turns out they're actually spawned in by something called entity_spawn_manager, which just uses the entity_spawn_point entities as a guide for where to put them. So that explains the respawning mechanic, although I still have questions — does it avoid spawning one if a player is standing where it's supposed to go? Will it spawn one in if a player can see it? In all the years since Harvest was added, I've never noticed either of those things happening.
Also, it seems there's only one parameter, the type of entity to spawn. How do I make it spawn something that I've set the parameters of, like what model to use and how much damage it does? Would point_template be of aid in this case, somehow?