The effects are separate .nif files, plugged in by the game when the animation plays. They are defined within Civ4EffectInfos.xml.
The position is determined by a dummy EFFECT_MISSIONARY_ACTIVE_CRISTIAN (or so) within the units .nif file. The name here does not matter, it's a placeholder;
Which effect actually plays is set in the ....activate.kf animation file. If you load it in Nifscope, you have to find a NiTextKeyExtraData node, open it, then open the Text Keys nodes it has inside. One of these Text keys will be something like this:
EFFECT_MISSIONARY_ACTIVE_CRISTIAN:EFFECT_MISSIONARY_ACTIVE_CRISTIAN
Where the formatting is NAME_OF_THE_DUMMY:NAME_OF_THE_ATTACHED_EFFECT
So you will be wanting to have something like this:
EFFECT_MISSIONARY_ACTIVE_CRISTIAN:EFFECT_MISSIONARY_ACTIVE_DRUID
And have a EFFECT_MISSIONARY_ACTIVE_DRUID properly defined in the said EffectInfos.xml.
I might be forgetting some detail, but in general it's how effects work.
You can also take a look at
Jarm for reference, we had effects working there (and one for Druids as well).