I put together a method for getting Fusion contacts in the Zoiper desktop client. I don't know if this will make it into the official source code, but I thought some people might want to take a look.
https://github.com/konradSC/Fusionpbx-Public/tree/master/Apps/zoiper_directory
https://github.com/konradSC/Fusionpbx-Public/tree/master/Apps/zoiper_directory