I doubt this works...but if it does rock on....

I have Sabertooth trigger his ability to pay 2 endurance to allow Character B to attack the hidden area.

Character B is in the backrow and does not have range. Does Sabertooths ability allow that character to still attack hidden characters?

I wonder because there are cards that allow a character to attack protected characters but doesn't grant flight.

If a character does not have range, it cannot attack from the support row. Sabretooth allows a character than can attack to attack the hidden area. Sabretooth's ability does not get around flight or range restrictions, just not being able to attack the hidden area restrictions. If you wanted to attack a protected hidden character, then the character Sabretooth gave the ability to would have to have flight or some other method of attacking the support row.