did a quick modeladmin for this home page caption dataobject to test it out. Got a getCMSFields function returning my form fields, and the LinkToID works fine here. The previous case was inside a pop up off a tab on the home page...
Cough Cough, yip thought so... it was me.
I'd set up a has_many "homecaptions" relationship off the home page. But didn't set up the has_one "homepage" off the caption class. Works a charm now.