I saw this happening first hand in Singapore 9 years ago. I was really surprised at the time. They were not really hardened criminals but Illegal immigrants (mainly Bangladeshi) bussed in every morning spending their day in shorts and thongs with a bucket of MEK and a rag, cleaning stringers in the cargo bay of D Check aircraft. At the time I was told they are made to do it for "a couple of dollars an hour" until they have earned enough to pay their flight home.
It was quite sad really. I saw so many of them washing themselves in MEK.
There were of course no QANTAS aircraft there then but I'm not surprised in the least it still goes on.