Well, I took a chance and setup tuner 0 and Tuner 1 as "Tuner Stream", and it does then find the ATSC 3.0 stations. Mapped (copied) the guide data from the corresponding ATSC 1.0 stations, and was able to record and play live tv (on the Server). However, live tv CPU usage was incredibly high at 75% (probably due to HEVC + madVR 1080->4k). Playing back a recorded show was somewhat less at 50% CPU. Normal tv playback (mpeg2 + madVR 1080->4k) is around 25% for me.
Playing live tv on a MC client PC is a disaster, I can see the tuner starts streaming by viewing the HDHR web utility (and a network monitor app on the server), but the client never actually starts playing audio or video, and just appears to be unresponsive. After pressing stop a few times and then closing MC on the client, the Server shows the tuner status as "Initializing" and I have to manually reboot the tuner and restart MC on the Server to get it back to "Idle". Playing the ATSC 3.0 recorded show works, CPU is normal because no madVR, but I get no audio (AC-4 issue).