My cam gives me jpeg with chroma sub-sampling 4:2:2, but I need 4:2:0.
v4l2 itself provides a very thin layer around the actual video data that is transferred: it will simply give you the formats that the camera (the hardware!!) delivers.
so if your hardware offers two distinct formats, then there is no way that v4l2 will offer you anything else.
you might want to checkout out libv4l2 library that does some basic colorspace conversion: in general it will have conversion from most exotic hardware formats to a handful of "standard" formats, so your application does not need to support all formats any hardware manufacturer can come up with. however, it is not very likely that these standard formats include a very specific (compressed) format, like the one you need.