No default VLAN as far as I can see. Ports are "untagged"
In my case (RB3011) different ports are "grouped" into a bridge-group and that bridge has an IP that functions as the default-gateway for my devices on the LAN.
So sure it can work with any other router connected without VLAN's, it will be just a port and with the correct routing in place it will work.
Please clarify what you are trying to build.