Find item by original filename with Exchange Web Services

Question

We used to have exchange 2003, and my webapp used webdav to create appointments in it. When an appointment was made I gave it a filename (whatever.eml) with a unique id that matched the database entry in my webapp for that event, so if I needed to later edit or delete it, I could easily do so by using the id to get the filename. We have since upgraded to exchange 2010 and I have to now use exchange web services and CreateItem to add a new appointment. I can simply store the id and changeid of the item when they're created to use to update or delete them later, but for the older appointments created via webdav, how do I refer to them (to update, or to find so I can get all their ids) when all I have is the filename?