I've been investigating the YUV overlays used by Picasa and it seems like it would be a huge problem to tackle this sort of support without some sort of external YUV conversion library and/or hardware support.
SDL has great support for transparent hardware/software YUV support; it might be possible to adopt some of the YUV hw/sw implementation code from SDL into Wine. Both projects are LGPL.
Whatever the implementation, getting support into the DirectDraw layer for scaling YUV surfaces is a fairly large job.
Matt.