I am looking for the same thing. If you come up with a solution please let me know. If I find something that works I will send it your way. I would be satisfied if it just told me that the file was in use but returning the USERNAME of the person who has it in use would be even better. Thanks.
I once wrote a program just for this. You need admin rights though. I will attach the project. If you put a shortcut to the exe in your Sendto directory, you can start is easily from explorer by right clicking on a file, and send it to this application. I have added some functionality to send a network message to the users.