To get the bounding box of just the tag you have to start a temp transaction and turn off the leader, then get the bounding box, and roll back the transaction. If that sounds like a lot of work it is. Doubly so when you can just pull the tag head position as shown here: Structural Rebar Tags need to align Align - #7 by jacob.small
Usually this will be the center of the tag - but there may be some variation for custom tags which were poorly built.