I've not experience of that I'm afraid, but wonder if you're making it too difficult for yourself......can you not 'simply' apply the texture (or a similarly pixel-dimensioned reference grid) once again in SI|3D, manually duplicating the positioning on-object ?
overlaying the image on the original in photoshop....
Erm, maybe this is a dumb question, but is the picture a '.pic', just in my very limited experience of XSI I notice it uses multi-format image files.