dw_dev | LogJam porting question: prop_xpost

I'm working on an update to Evan Martin's LogJam client to support the metadata and tag changes on Dreamwidth. I have it mostly ready to go, apart from a few site-specific issues like the cut and user tags, but I'm running into a problem with prop_xpost_check. When I submit a test post that attempts to set the prop_xpost_check property, the submission fails with an 'invalid metadata' error. Is the auto-crossposting feature intended to be usable by remote clients, or is it available only when submitting through the web interface?