Is it legal for T-beard to deal X damage to himself and then negate it all with one Close Call card?

We've had rulings about this in the past-- see the thread here . If you use Treebeard's damage and then cancel that damage (via Close Call, Honor Guard...), Treebeard does not get a bonus.

Also, Treebeard can only deal damage 1 point at a time. His Action doesn't say "X," it says 1-- you can trigger it multiple times in a row, but you're still dealing only 1 damage per instance. This means that Close Call can only cancel 1 point of damage from Treebeard's ability, since you can only use Close Call for one triggering of Treebeard's action, and then it would be discarded.

Effects like this should be understood to read

Action : Assign 1 damage to Treebeard. If this effect places damage tokens on Treebeard, give him +1 [Willpower] and +1 [Attack] until the end of the phase