IIRC this was ruled somewhere. The reason that I remember is that Kagi only has you lock onto him "if able", and you're not able to lock onto Kagi if you have to lock onto something else.
This is exactly correct. You only have to choose Kagi of he is a legal choice. With Tarn's ability you only have 1 choice, the attacker, therefore Kagi is not a valid target, so his ability does not matter.