If it doesn't support YUV4:2:x it won't be able to do 10-bit 2160p60. If it can only do 8-bit RGB 2160p60 HDR playback is going to be gimped.
However, I don't know if the Videocore VI is capable of decoding YUV video, leaving it in YUV color space, and rending the Kodi GUI/OSD either directly into YUV color space or rendering it to RGB and converting it to YUV for blending and output so that the video can be passed unmolested without two color space conversions.
The Pi Devs have said 4:2:2 support is there - but it looked like they suggested the vertical subsampling for 4:2:0 meant that wasn't an option. If that is the case, then that means 4:2:2 12-bit will be the only HDR HDMI 2.0 output mode for 2160p50-60 supported I guess (as there is no 10-bit 4:2:2 defined for HDMI 2.0 - so 10-bit HDR HEVC content is output at 4:2:2 12-bit not 10-bit)
For 2160p24-30 content you can run RGB or 444 and output HDR, without needing 4:2:2 (though 4:2:2 12-bit is valid at these frame rate resolution / combos as well, whilst there is no 4:2:0 support in HDMI 2.0 at <50p). For the bulk of HDR HEVC PQ content - which is 2160p23.976 movies an tv drama - RGB or 4:4:4 YCrCb output in 10-bit is a supported HDMI 2.0 mode and is fine.
I guess if you render Kodi to YUV-space you'll need to have both Rec 709 and Rec 2020 RGB->YCrCb conversion implemented, unless you mind GUI rendered stuff being a bit out (are photos gui-rendered)