After waiting another 4 hours for the next SharedCoin transaction, which was still unconfirmed, I googled a bit and came up with this post, where someone just 12 hours ago had the same problem and got the coins unstuck by rebroadcasting a transaction manually to another wallet:

http://www.reddit.com/r/Bitcoin/comments/1y7omy/heres_some_good_tools_to_know_about_in_case_you/

Maybe the information/method contained in that post will come in handy for someone.

He points to one transaction with an output < 0.01 BTC and a size > 2KB with no fee attached somewhere down the SharedCoin transaction chain as the culprit. Whether that is a problem in the SharedCoin implementation as suggested in that post, I will leave for others to decide.

I didn't get around to trying his approach to solving the problem anyway, since while writing this post, the transaction finally started to propagate and I expect it to be confirmed soon

So I didn't investigate, whether the reason for the delay in processing was the same or similar as described in that reddit link or whether there's another legit reason for the delay in propagation (which sits there at 0% for hours and then suddenly rises quickly). Maybe it's just normal and to be expected waiting for inputs to be confirmed (which still show as partly unconfirmed, while propagation is rising, though - it appears as if blockchain.info is waiting all the time, before it finally broadcasts the transaction in question. Since some inputs are still unconfirmed, while propagation starts, I don't really know for what.).

Just sharing my experiences...

Maybe someone more familiar with the intricacies of CoinJoin and blockchain's implementation of it can fill in the gaps