Basically I have a spell the player can fire and I want to detect who the target is so that I may bestow a certain buff on him when the spell effect ends. The spell is single target touch. I know how to do it using OBSE, but is there a way to do it without using OBSE (and no, I don't want to hear replies such as "Use OBSE", "What's wrong with OBSE?", etc.)

You could use the initial spell to add a token to the target. The token can use a timer and just add the buff after the initial spell ends. Or, it could be set up to use http://cs.elderscrolls.com/constwiki/index.php/IsSpellTarget in some way.

This is really simple. You can either use the ScriptEffectFinish block on the spell script (which runs when the spell finishes) or add a scripted token to the actor, as Vyper mentioned.