This can be solved by hosting the nested Detail Item Family to the face of the Extrusion instead of to the Ref. Level. First, adjust the extrusion so that the bottom face of the fixture extends beyond the host face. In the Ref. Level View, select the Detail Item Family and with it still selected, navigate to a 3D View. Select "Edit Work Plane" from the Ribbon, choose "Pick a Plane", Click "OK", and then select the appropriate Face of the Extrusion.
Sorry for the delay; here is an example family in Revit 2012 format using the default Revit recessed light fixture Family.
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