People used to buy BfA on a second account (on same battlenet account) for a char boost when they were account wide, they are changing that for SL, but I have no idea if that means the old boosts no longer work.
This is the change for SL
In-game items and Character Boost not available in World of Warcraft® Classic, and Character Boost usable only on the WoW game account on which it was purchased or redeemed as a gift
It’s not a method I’ve ever used myself. You could ask for clarification in the Customer Service forum.