Well I would not want to be doing that trip either – sins the pictures are taken from 400km away from Reykjavik – the long drive would not allow for much climbing in a day tour 
But if you are in Skaftafell you might want to drop in with the IMG guides in the area and see if some of them are heading out to Hnappavelli in the evening – other way there should be people in Hnappavellir all the time if the weather is good.
Good luck