Thanks for reaching out. A quick status update. We are still working hard on the layout engine upgrade, it is one of our important development projects. Unfortunately things take longer than expected and no official eta has been set.
The grayscale topic (or filter property) is two fold. Support in the layout engine is only one part, we also need to apply the conversion when generating output. At this stage the only workaround I can think of is converting the images on beforehand. This could be done with a tool like ImageMagick:
convert C:/source/*.jpg -set colorspace gray -set filename:base "%[basename]" C:/out/%[filename:base].jpg
Hope this is of some help,