Are there any mainstream books that feature a female protagonist humiliated? Preferably humiliated in a WAM way, but I am open to other forms of humiliation.

Not sure whether it qualifies as mainstream but Love A Messy Business by Abbie Walton has a humiliation WAM scene in it - I don't have the link handy but you can find it on Amazon. It's a romantic comedy so you'll have to be patient to get to the messy stuff!

I've found some in the Penny Birch books

'Tight White Cotton' http://www.amazon.co.uk/dp/B003GFIVQ6 is a collection of short stories, at least one of which includes mess, which I think was tar.

'Tie And Tease' had some play in mud http://www.amazon.co.uk/dp/035233987X

'Penny in Harness' had some interesting mess too as I recall, but mostly it was incidental rather than the centre of the action. http://www.amazon.co.uk/dp/0352332719

In looking for these (I could not recall the titles) I found 'Uniform Doll' http://www.amazon.co.uk/dp/B003F2QNYW which suggests it ends in a sticky mess and 'In For A Penny' http://www.amazon.co.uk/dp/B003RWSB90 which mentions baked bean wrestling.

I think I need to go shopping.

There are also some very good stories on www.fetlife.com like https://fetlife.com/groups/8358?page=5 you'll need to join to read but it's free and quite fun.

Thanks for the suggestions, folks!