[jeruku]

Without a proper UnitID(i.e. target, targettarget, party1target, nameplate1, etc.) and nameplates you'll have to use the combat log method. As a plus every unit has a UnitGUID which can be tracked in the combat log.

Since you need a valid UnitID before casting/attacking you can get the UnitGUID and health on an event related to casting/attacking. Once you have those two things you can keep track of damage and healing from the combat log for a focus. This however means you can't click the focus frame since it has no UnitID, though you might be able to modify the interface to use something like /targetlasttarget (it is not a valid unit token) to constantly update the last target to always be your focus.