I don't think you can. If you go to see your purchases, by each purchase there should be a "download album" link. Next to one of my purchases instead there is a "more info" link and if I click it I get the following message: "Sorry, {album name} by {artist name} is no longer available. Please contact {artist name} for more information."
But is the download link also missing from your collection page?
edit: I found one track that has the "more info" link in my purchases, searching for it on my collection gives no results. I guess that answers that :(
It's pretty incredible that in about a year they'll hit $1000m of payouts to artists.