Obviously this will be limited by how many HDHR's or streams you can play, but if you have enough, it would be really nice feature!
Emby, Plex, Jellyfin, nor ChannelsDVR support custom multiview. I'd love to be able to use the mobiel app to quickly group channels into various multiviews (between 2 and 4 channels). FFMPEG can do this, I was able to prototype using this, but its not probably 100% right. However, it really needs to be in the client app to be able to quickly do it in the interface for sporting events or whatever you want to multiview.
Here is an example of it with Tivimate - https://www.youtube.com/watch?v=k8v0FO-MBB4
ffmpeg -i stream1 -i stream2 -i stream3 -i stream4
-filter_complex "nullsrc=size=1280x720 [base];
[0:v] setpts=PTS-STARTPTS, scale=640x360 [upperleft];
[1:v] setpts=PTS-STARTPTS, scale=640x360 [upperright];
[2:v] setpts=PTS-STARTPTS, scale=640x360 [lowerleft];
[3:v] setpts=PTS-STARTPTS, scale=640x360 [lowerright];
[base][upperleft] overlay=shortest=1 [tmp1];
[tmp1][upperright] overlay=shortest=1:x=640 [tmp2];
[tmp2][lowerleft] overlay=shortest=1:y=360 [tmp3];
[tmp3][lowerright] overlay=shortest=1:x=640:y=360"
-c:v libx264 output.mp4