actually what you describe, is to connect a 100/10 connection without poe to mikrotik. this will make a port/interface active. as loopback, it would just connect to itself. You can use this port, to a lot of stuff. Loopback interface, internal, is one thing, but loopback in Ethernet is also possible. but why do you want to do this? If you want. you need to revert rj45 connection, so that RX and TX in both sides is the opposite. eg. 1-3 and 3-1 and 2-6 and 6-2)
If you buy a crossover cable, it will do the stuff 4 you.