I managed to run jack2 backend -net on Linux (Parrot Home =Debian Buster rolling) and jack2 -net in Windows 7 VM (Qemu/KVM) together.
I suppose that Linux jack with jack_load netmanager seems to listen for multicast only on device eth0, but not on VM device virbr0.
I found a workaround with installing smcroute (static multicast router daemon), configured to mirror multicasts from virbr0 to eth0:
phyint eth0 enable
phyint virbr0 enable
mgroup from virbr0 source 192.168.122.105 group 184.108.40.206
mroute from virbr0 source 192.168.122.105 group 220.127.116.11 to eth0
where .105 is the VM IP and .154 the default jack multicast IP
Now the Linux jack recognizes the VM jack and connect to it automatically.
So I can use VM MIDI and audio ports and play Windows VST plugins, with MIDI-input from Linux host and VM audio-outs back to Linux host, and low-latency (with option -l1). Nice!
I still have a problem with jack transport. Jack on Windows says "Netdriver started in async mode without Master's transport sync".
So when I start transport (= playing) from Linux DAW or qjackctl, with the VM running, I get a delay about 5sec until Linux transport/playing is really starting. Windows transport is not starting anyway.
Seems that the Linux master transport is waiting for sync with VM jack until timeout. If the VM is not running, no delay happens. Asked about it in JackAudio mailing list (May 2021), but no answer so far.
I tried backend -netone, but got initializing error "netjack_poll not implemented ... initing net driver fails" with Windows jack. so the Linux master could not connect. Happens with Windows version jack 1.9.11 and jack2 1.9.17/18, seems to be a known problem.