[DeltaMasterMind]

This would be one hard feat to make with that exact wording.
Doing the additional Favored boxes for class is fairly simple as a script with the above mentioned perform tag script like so:

Timing Guessed at Post-levels 5000.
field[abValue].value += #totallevelcount[]
~ If we're not shown, just get out now
doneif (tagis[Helper.ShowSpec] = 0)

~ If we're disabled, do nothing
doneif (tagis[Helper.SpcDisable] <> 0)

~ add that many HD-counts-as tags to the hero
var i as number
for i = 1 to field[abValue].value
perform hero.assign[Hero.Multitalen]
next

"you may select one favored class benefit from any character class"

Now depending on how this is read makes this simple or hard. If any class you currently have then no further work is needed, but if it means any class available class in herolab than a lot more tag counters are needed and I honestly have no idea how to do this.

Edit: Seems after looking into it a bit Rogue Genius Games has that feat yet no code in it. So the above script will give you a new favored class to select, even though you can pick any class as a favored class, you are only allow selections of bonuses on the class you currently have taken. So basically the trick would be to make this feat see all class tags as active and valid if we are to make any favored option available from class level you have. As before I am not sure how to do this. Racial favored class options should be good though.