Right, I too doubt the usefulness of standard UVC driver for your application.
Better late than never...
I have grayscale video working from a FX3 to a Linux box. The Linux uvcvideo driver supports the 'Y800' FOURCC.
Windows is not so accommodating. There, I think you would have to register your own codec to interpret the video data.
I believe you may need a filter driver in addition to UVC driver for this purpose.
Its been a while since your 1st post perhaps you have answered your question already. There does seem to be little web discussion on the subject on UVC, YUV and greyscale.
Anyway I have developed a UVC camera based on an Aptina sensor specifically for greyscale applications. The sensor normally outputs color but changing a register setting forces it to output only luma information which actually works. Exactly what this means data-wise isn't discussed in the datasheets but it could just be the 'Y' byte (Luma) with the U and V set to 0x80 i.e. zero chrominance (probably easier to quote than arrange)
Interestingly AmCap by default shows a "ColorEnable" tick-box but there is no mention of it in the USB specs or web-discussions, if anyone has the answer please let me know, for the time being I'm using a Vendor Command to access this control.