Have been working on this today, have it partially working (crashes still a lot, the append part of the code is very complicated :( ), but am hitting some design issues actually.

Thing is, in append, after linked data has been made local, we remap all its local usages to the local data-block. It means that if e.g. you have assigned a linked material to some local object, then decide to append that same material, currently your already existing local object will be also remapped to use the new local copy (appended version) of the material. I kind of have the feeling that this is not the desired behavior?