very simple, my friend haha. When going from FCP to Shake, you are offered an option of where to save the "placeholder" file. the placeholder is going to be put in your final cut timeline in the very top track (adding a new track every time you do this). When you are in shake, they give you a fileout node already directed to the location of where you saved the placeholder file with the same codec as your FCP timeline. build your composite in shake, select the fileout node, and render.
after the render go back into FCP and your placeholder file should be in your timeline directly above the stuff you sent to shake (on the very top track) but the track it is on is just turned off. simply turn the track on and its ready to go.
sorry for being this insanely late haha...i was busy, but i know this has worked for me since FCP 5 and shake 4. let me know if it doesnt work and i'll tell you how you can do it another way to avoid rendering in the timeline of FCP