I was recently in contact with Wakaama  community (part of the Eclipse IoT foundation), and which maintains an open source implementation of LWM2M.
More to the point, they told me that the code base  is deprecated (it was their repo prior to joining Eclipse), and that their supported codebase is now .
They would be very happy if their implementation would be ported to RIOT, and would provide some support for that (they suggest whoever is interested in that contacts them through ).
I thought this information may be of interest for us, and in particular for GSoC project proposals on this topic.
 https://github.com/eclipse/wakaama  https://github.com/01org/liblwm2m  https://dev.eclipse.org/mailman/listinfo/wakaama-dev