We had a similar use case in the past, need of a dummy interface. We solved this with a new VLAN interface on an existing BOND. To get the interface up you have to configure the connected switchports with the same VLAN-ID, because they are monitored as you wrote. Normally only first and last VLAN on a trunk are monitored, you could use one in between without to be monitored . But for troubleshooting and some other interaction it's better to configure this like a normal, including the switch.
If you can't use a VLAN you can use a new physical interface but you have to connect these to a switch to get the link up or if only two gateways are used via a direct attached cable.