Bumping for you. Ds used to bite his wrist/hand too - he still does occasionally. Prior to that it was usually me and I used to tickle the back of his neck because he has a very sensitive spot (he has an angels kiss there) there and usually opens his mouth and goes slightly floppy! I think it is unique to him though so not useful to you.
I have heard that Chewy Tubes which children can bite on can help. There's some info about this and very useful advice re self biting in general on the The National Autistic Society website(www.nas.org.uk) The Chewy Tubes can be bought via Kapitex 01937 580211.