LMS 3.0.1 and higher can discover through MPLS. Prior to LMS 3.0.1, Discovery was only CDP-based, and thus did not work with MPLS. LMS 3.0.1 introduced new Discovery modules such as ping sweeps, ARP table lookups, routing table lookups, etc. Assuming one of these will work in your topology, then you should be set.
To configure Discovery, go to Common Services > Device and Credentials > Device Discovery. You can then choose as many of the available modules as you like, but it is preferable to choose as few as possible to fully discover your network.
