While I was reading the first half of the question, I was already thinking that I'd throw a picnic tap on it. That way you could also pull out the sludge before moving it. If it's been sitting a while, you probably have a bit of yeast cake and you could get rid of some of it just by tapping it. Then, dump that and have a sample!
I don't see any problem with taking out a sample with a wine thief, though. You could open it, take the sample, close it and purge it a few times. I think the picnic tap is easir, though.