Where can I find XScaler? A Google search does not seem to turn up anything relevant. I'm not holding out much hope, though... the problem with the chroma subsampling is on the output side of the conversion, not the input side.
I also tried Quicktime 7.6 (with it's unspecified improvements to H.264 single-pass encoding quality) to no avail. Doesn't look like 4:4:4 is all that ubiquitous for H.264, despite the format supporting it.