You can buy a cdkey for any old Blizzard game for like $5 online, that will let you register your product and get an opt-in for the beta. Assuming you care that muchOn top of that you can register any valid CD key with a Battle.net account on the webpage, and that will allow you to download full install files for any Blizzard game that you register. Worked out great when I wanted to reinstall Diablo 2 on my shitty laptop that won't run anything new, but my original Diablo 2 CD is more beat up than "Shamwow" Vince's hookers.
I don't have any blizzard games so even though I created a battle.net account it told me that I needed to register a game before I could apply to the beta.