How can I see the cast bar of my focus target?

[Zetonegi]

What about a Focus cast modifier?

For example, if I set a key (let's say Shift) and use an ability while holding Shift, the ability will be cast against the Focus target instead of my actual target.

Focus is good, but without a Focus cast modifier it has very little use.

A macro of
/macroircon "Ability"
/action "Ability" <f>

will use the ability on your focus target.
This, of course, requires another hotbar set up with all these macros on it or you can use a redundant macro of

/macroircon "Ability"
/action "Ability" <t>
/action "Ability" <f>

or

/macroircon "Ability"
/action "Ability" <f>
/action "Ability" <t>

depending on if you want the macro to prioritize your target or your focus target. It will try to do the first line, if it can't, for example invalid target, it will execute the 2nd line, which if you execute the 1st line it won't do because cooldowns. You might want to filter errors in whatever chat log you primarily use though.