Question - How would you configure a NUC (or other PC) so that it can be used as a NAS to serve files to other HTPC, and so that it can run XBMC?
Just enable
file sharing (wiki) on the OS. Even OpenELEC has file sharing options.
Ok... so could you do this:
Box1: OpenElec + Media files + WakeonLan
Box 2: OpenElec
Box 2 wakes up Box 1 and plays movies stored on Box1?
Yes, if Box 1 supports WOL.
After you price out a NUC plus some kind of mass storage you are going to be at the price of a decent NAS so just do yourself a favor and get a NAS for file sharing.