There are several MiniPCIe cards with one or even 2 GigabitEthernet ports, from different manufacturers. Intel, Realtek, Broadcom…
Would be possible to use a card like that on a Routerborad like RBM11G or RBM33G?
That wolud be recognized by RouterOS as an Ethernet Interface?
There would have to be drivers for it in ROS for that platform. Without drivers you will see it in system/resources/pci but that will be all.
Unlike x86 ROS that includes drivers for most common hardware (so intel/realtek cards work) builds for other architectures are slimmed down and doesn’t contain any extra drivers.
But there’s a good chance it might be added in some future version, as kernel modules are usually just a few kB in size… so if there is demand from users and enough people keep nagging support for long enough it might be added…