I discussed this with dcamp over email. The issue is that chrome expandos on content objects go on the xray wrapper expando object, which is per-compartment. So with cross-global access (ie, outside the JSM), the inplaceEditor expando is invisible.
Patch coming right up.
Created attachment 609508 [details] [diff] [review]
Access the inplaceEditor expando with an explicit API. v1
Attaching a patch. Flagging dcamp for review.
Pushed to m-i: http://hg.mozilla.org/integration/mozilla-inbound/rev/d211c6492297