The effect of Norn's Annex can be interpreted as follows:
- Creatures can't attack you or a planeswalker you control.
- A creature's controller may pay (W or 2 life) at a given attack step for that creature to be exempt from this restriction for that attack step.
So as far as the game is concerned, Goblin Brigand is not able to attack. The fact that a player could choose to pay a cost which would make Goblin Brigand able to attack is not relevant.
The comprehensive rules that support this interpretation are
508.1d ... If a creature can't attack unless a player pays a cost, that player is not required to pay that cost, even if attacking with that creature would increase the number of requirements being obeyed.
and more generally,
117.12a Some spells, activated abilities, and triggered abilities read, "[Do something] unless [a player does something else]." This means the same thing as "[A player may do something else]. If [that player doesn't], [do something]."
I think it's a fairly general rule of thumb that players are not required to pay costs to make things possible.