Is it a bad idea to send an article under review as a writing sample? (faculty job application)
First, From a legal point of view I do not think you will have any problems sending out a pre-review version of your manuscript since your have the intellectual rights to your own work. Usually most publishers' copyrights start when they have processed the material in some way but even then, if you are not using the material for any commercial purpose or for being reproduced publically, I cannot see any problem. 

Then, I cannot really see any problems sending a pre-reveiw manuscript to a search committee unless it is of poor quality. Sending your work allows people to see what you are doing and a manuscript (in very good shape) will allow people to see your latest work in detail. Naturally a manuscript will not carry as much merit as a published paper since it has not been through peer review. But, if it is interesting, well written and in impeccable order you will be showing your skills in some way. 

So it could provide a slight positive in a search situation if good but be very negative if bad and your judgement is what will be at stake.