The script /usr/libexec/nethserver/nic-info is responsible, this skips my dummy interface.
I modified it in a way that it let pass interfaces of type dummy (see below). And at least initially this does not seem to break anything: no obvious warnings/errors/mis-behaviour, I could successfully configure it to belong to a new bridge - but tests are pending, I will continue next day(s).
UPDATE: due to strange effect I had to change the script not to drop other virtual NICs. As with doing so, my dummy can no longer be configured but deleted only. Letting pass the 2nd NIC, too, everything looks fine again. Realy really strange.
So below the most recent patch to the script is shown.
Fyi updated nic-info diff:
57,59c57,64
< if ! [ -e /sys/class/net/${card}/device ]; then
< continue
< fi
---
> # NO IDEA, WHY, but with passing the dummy interfaces only, they cannot be
> # configured but deleted only. Letting pass the 2nd virtual nic things
> # can be configured again ... strange, very strange.
> # if ! [ -e /sys/class/net/${card}/device ]; then
> # if [ -z "$(ip link show ${card} type dummy)" ] ; then
> # continue
> # fi
> # fi
79,80c84,89
< driver=`basename $(ls -l /sys/class/net/$card/device/driver | awk '{print$NF}' )`
< type=`basename $(ls -l /sys/class/net/$card/device/subsystem | awk '{print$NF}' )`
---
> if [ -r /sys/class/net/$card/device/driver ] ; then
> driver=`basename $(ls -l /sys/class/net/$card/device/driver | awk '{print$NF}' )`
> fi
> if [ -r /sys/class/net/$card/device/subsystem ] ; then
> type=`basename $(ls -l /sys/class/net/$card/device/subsystem | awk '{print$NF}' )`
> fi