In tundra, you have these awsome mammoths outside the valiance keep. What I did with my 70-76 paladin was to hit 1 mob, and the respective pack will follow. You strafe into a second group, and you throw down conc so the mammoths will keep follwing you. You can do this in a full circle, constantly killing off mammoths, never going out of mana (BoS / divine plea), and never dying. With rested exp I did 1.2 mill / hour, half that non rested.

What you will need (or what I used)
Glyph of righteous hammer - hits 1 additional target.
A slow weapon with lifeward enchanted.
And glyph of divine plea.

How to execute: Judge a mob with judge of light before you hit hammer of righteous. With lifeward and seal of light this will net you about 2-3k hp back.

Armory for gear reference: The World of Warcraft Armory

Its not about doing tons of damage, its about surviving indefinitely while the constant stream of mammoths slowly dies off

I'm thinking it might be to prevent level 80's from queuing up for old-world instances and massively trivializing it for the 4 other level 20's trying to legitimately run Deadmines.

Or does the LFG system automatically prevent you from queuing in greyed-level instances anyway?

You cannot queue for any level-inappropriate (as decided by Blizzard) instances. Lvl 80s are even shut out of normal Northrend instances that are "too low", e.g. Nexus.

With the most recent patch, you no longer need Battleground Tokens to buy PVP gear. Have you main send your alt a Wintergrasp Commendation, redeemable for 2000 honor. The level 28, 38 and 48 boots and belts cost 300 Honor each, and are pretty good for their level.

The daily reward for participating in the "call to arms" is pretty high right now. As I leveled in the 55-62ish range a couple of weeks ago it was rewarding about 1/2 a level for a win, on top of any XP gains during the game.