Ok, this worked with Zul 'Gurub and Sunwell Plateau so its gotta work with others. When trying to summon a player too low level for an instance with the summon stone, it doesn't let the player enter the instance because the spell the command casts when you right click has a level restriction, just like the one that doesn't allow you to have a level 60-70 buff casted on you. The instance portal works the same way, with a spell that teleports you. The way BGs rectrict you is using different teleport spells, but if a spell that has no restrictions, such as the *Warlock's Summon Spell*, you can get teleported anywhere is the game! When you enter an instance or a bg and have a warlock and two party members with him/her (two party members of the same realm in a bg) and the warlock uses the summon spell while in the instance the person gets summoned right in.
Abusing this bug to make twinks with level 70 equipment:
As many of you know, there are level 70 items that have no level requirements. So... who wants to beat the crap out of the battlegrounds? There are very few no-level requirement that drop, but there are many no-level-requirement items that can be bought from vendors. For example, if you bring your level 10 to sunwell plateau and get reveared with Shattered Sun Offensive, bring 15 gold to the island and get an insane supposed-to-be level 70 sword, or even get two and duel wield them for 30 gold, so when you level up to 19 there will be no battleground that you will loose... besides which you will be getting incredible honor from that high DPS.