No, I don't think releasing a single would be a good thing. The DVD need to be promoted instore, not by a videoclip (big chance it won't be played on MTV). You don't want to overstock the musicshops even more.
I think the big hit would be when BMG started to realise that 'the new audience' (youngsters who haven't heard of Elvis) aren't as crazy about the 50s Teddy Bear/Don't Be Cruel Elvis as the youngsters in the 50s. I think BMG could sell the '68-'74 Elvis way better. Not only the music from that period, but also the pictures from that era.