There's an addon, PandaDisenchant.
It opens a window with all your DE'able gear in it, and anything you click on is disenchanted.
You can set a threshold, and anything above that level is not put into the window.

You still need one click per each item to be disenchanted.
If you turn auto loot on, its a much faster process.

I was doing this for quite a while on one toon, who was making gold via arbitrage.

I had an excel spreadsheet, with the value of each type of dust, shard, essence or crystal entered.
And it would look at armor by item level and weapons by item level, and multiply in the values of the DE mats.
So if something was:
85% chance of 3-7 dust, 14% chance of 1-2 shards, and 1% chance of a void crystal...
I would calculate:
0.95 x ( (0.85 x (3+7 x Dust Value)/2) + (0.14 x (1+2 x Shard Value)/2) + (0.01 x (1 x Crystal Value).
The 0.95 at the start accounted for the auction house cut.
And with no list fee for enchanting mats...

Well, unfortunately I can attest there is no achievement for either 10, 20 or 25,000 items disenchanted.
But it was a pretty easy 350g an hour, even if it was rather boring.
At least I had all of Stargate SG-1 and CSI (Vegas) on DVD as distractions.