The only time library parts were backward compatible was from 7 to 6.5. One way to do it is to remake the part in the earlier version by copy/pasting the scripts. You will have to rebuild the parameter list manually though. I seem to recall reading that it is possible to export the part(s) as XML from one version and import that into a prior one but I am not sure about this.
1. Add the Library Object to the Embedded Library in any AC14 file
2. Save As>ArchiCAD 13 Project
3. Watch the progress meter ... you should see one of the stages is 'Converting Embedded Objects' or something to that effect
4. Open the resulting 13 Project in AC13
5. Extract the object from the Embedded Library to wherever you wish
6. (Hopefully) Voilà .... one AC14 object converted to AC13