Just to provide an update here, we updated both jellyfin and emby addons in the linuxserver repo. They now both support omx hw transcode.
Go into the addon settings and toggle openmax. When the container is recreated, necessary devices and drivers will be mapped in.
One gotcha though, neither supports hw decode. They only do hw encode. According to jellyfin documentation, that is an upstream limitation: Hardware Acceleration | Documentation - Jellyfin Project
I also tested v4l2 (everyone says it's the future). It is detected and used by both emby and jellyfin (again, only for encode) but unfortunately the image has a green filter over it. No idea why, so we didn't enable that in the addon yet. Let me know if you have any idea what causes it and/or how to fix it.