What is the enhancement in mind?
An extra parameter for substitution in the message format strings,
that will indicate when the source is "my focus".
An additional editbox in format option section where
one can set a text flag to a static string of their choice.

How should it look and feel?
To take the default "casts" format string as an example
it could be something like "$f$c casts $i$e"
where $f is used to substitute "my focus" flag. (for example "^")
So if my focused unit is the one casting it would output:
"^ Ammo casts Torch" or if it's not my focus it would be
"Ammo casts Torch"
(would help with general awareness as well as focuscast macro usage etc)

Please provide any additional information below.
In WitchHunt:COMBAT_LOG_EVENT_UNFILTERED
adding an extra flag
isSourceFocus = (bitband(srcFlags, COMBATLOG_OBJECT_FOCUS) == COMBATLOG_OBJECT_FOCUS)
and cascading it through WitchHunt:Burn -> WitchHunt:GetFormatted
As well I imagine that an if (target) elseif (focus) construct would
be preferable as you'd want only the target flagging if you happen
to have your focus targeted.