This is an extremely simple add-on that illustrates how to modify targeting behavior on action buttons. It allows you to shift-click any Blizzard action button to target that action on your focus.
There are two things you need to do to try and make this affect a bar mod:
Add the names or Lua identities of the action bar frames you use to the list in line 3 of ShiftFocus.lua.
Add an ## OptionalDependencies: line to ShiftFocus.toc with the name of the bar mod in it, to ensure that it loads before ShiftFocus does.
Please contact me directly (such as by PM) before uploading any derivative of ShiftFocus (or any of my mods) to any public website; they are not under a public license.
I hope you don't mind, but I plan on taking your addon and modifying it a bit; since I already have ways of dealing with focus targets, they aren't my issue.
What I really could use, especially on my warrior and feral druid, is an easy way to turn spells into mouseover spells with a modifier.
EDIT: I'm finding that I might need a bit more help than I realized to modify this... I use a bar mod, so I have no clue how to get it to recognize the bars (I just tried it using the main bar, Bar1, and it didn't work).