Premise: Set a monster's firing function as an info_notnull's use function to make it fire monster missiles of your choice.

Problem: All the monster missile functions require the self.enemy field to be set to the entity for which to aim towards.

Solution: Entity references in quake is really just a number indexing the entity in edict list. The player will always be entity 1. If you set the info_notnull's "enemy" field to "1", you'll get a entity that fires a missile towards the player when triggered. I'm sure you can, by looking through the edict list when you have completed the map for release, figure out what the entity index for any entity will be in your map, and so make it aim towards any entity.
For example, in the map I uploaded below, all the info_notnulls have "enemy" "7", which I found out through the edicts list is the info_player_start in this particular map. I think that your results may vary on this, as it could be that editors like to save the entity order in not exactly the same way every time, and also other uncertain factors.

Examples:
"use" "OgreFireGrenade"
Fires an ogre grenade towards self.enemy.

"use" "CastLightning"
Shoots a shambler lighting bolt towards self.enemy.

"use" "hknight_shot"
Fires a single (just one) hellknight magic missile towards self.enemy.
Requires a hellknight to already be present in the level for the precaches to work.

"use" "ShalMissile"
Deploys a voreball that starts seeking towards self.enemy. I had some problems with this when it collides with un-alive things. Looked like it just disappeared without a trace.
Requires a vore to already be present in the level for the precaches to work.

"use" "Wiz_StartFast"
"health" "1"
Fires two wizard slimewotsits towards self.enemy. The health field is required because the actual function that fires the missiles checks if the scrag is alive first. (See below.)
Requires a scrag to already be present in the level for the precaches to work.

"use" "Wiz_FastFire"
"owner" <entity-ref>
Fires a wizard slimewotsit towards self.enemy. The owner field is required because this function checks that whatever spawned the missile (usually a scrag) still is alive first. Thus the owner field here must refer to an entity that is "alive". That is, it must have a "health" field set to anything non-zero. "owner" "1" should work, as that refers to the player, which usually is alive.
Requires a scrag to already be present in the level for the precaches to work.

"use" "boss_missile"
Fires a Chthon fireball towards self.enemy. This requires that self.enemy is alive (has health), or else it will start seeking after the player no matter what you intended. I tested this with a info_player_start as the target, and it worked once I gave it "health" "1".
Requires a monster_boss to already be present in the level for the precaches to work.

The result:
http://czg.spawnpoint.org/stuff/ogretest1.jpg
http://czg.spawnpoint.org/stuff/ogretest2.jpg
(Sadly didn't manage to capture the shambler bolt)
Map file:
http://czg.spawnpoint.org/stuff/ogretest.map