I don't think that is going to be possible in LT. In order to get a field to read the value within a block yoou have to use the field dialoge box and the select object option. And in LT there is no way to bypas the dialogue box in a macro or script, you can start the field command from a macro but that isn't going to help you because you still need to physically click on the select object box, then select an object, then choose which property you want to use etc. etc.
What you should be able to do is make a block that already has your lighting block and a leader created with the necessary fields. And this block should inserted with the automatic explode enabled (you can do this with a macro). Then you can update the block info as needed and it's associated leader will update accordingly (after a regen of course)