In response to my own question, I found this tip over in a GenArts faq:
On FCP, the plug-in effect is cut off at the edge of the layer.
If a plug-in is applied to an FCP layer that is smaller than the final output, the plug-in will still only affect the area within that layer’s size, and sometimes appear to be cut off. To extend the effect beyond the edges, just make a nest from the layer and apply the plug-in to the nest.
One way to make a nest is to create a new sequence, drag your footage into it, and then drag this new sequence into your Main sequence to nest it. Then apply the plug-in to the nested sequence, and it should affect the area outside the original layer.
Is this really the most elegant solution FCP has to offer? I’d love to know.