Is it practical, or at least possible, ideally using the basic construction set, to detect when the player is trying to "yield," and who they are trying to yield to?

The idea being that anytime you try to yield to a hostile character, it reduces their aggression slightly, based on your speechcraft and personality.

Alternatively, is there already a mod that does this?

I don't know if there is already a mod that does so, but, with OBSE, it should be very possible. When the player is in combat, get his crosshair reference, and look for the key combination of blocking, and activating his opponent.

Cool. Is there a manual for using the OBSE? I've not had great luck in understanding it or its predecessor(?) in past attempts.

http://cs.elderscrolls.com/constwiki/index.php/Portal:Scripting is a good starting point, but, isn't completely up to date.

http://obse.silverlock.org/obse_command_doc.html should be more up to date, and is another good resource.

You will have to install OBSE, if you haven't already...... and 'adjust' your shortcut to the CS to enable it, then, it is as simple as writing the scripts you need right there in the CS.

Edit: Keep in mind, you have to run the game with OBSE as well, otherwise, your mod won't work.