I think that this spatial conform option is not completely finished. I am also trying to tackle this problem. Not for a real project yet, but without it, the application is even less usefull than it already appeared to be.
ProRes material that is run via Compressor with a 16×9 tag will work in an anamorphic project, but I tried the same thing with the IMX codec, because we have a lot of IMX material on our server and then it didn’t work. (I don’t want to re-transcode all our material anyway, but it was just a test to see if it would work).
When done with a ProRes clip, FCPX also did not have to render the timeline, just as in FCP 7 was the case. If you use distort it will need to render.
Dropping an SD PAL ProRes clip which has been transcoded with Compressor with a proper 16×9 tag will also scale correctly in am HD project, it will need to render for the upscaling though.