Keep in mind though, that even 10BASE-T works with high enough of a frequency, that you need to think about impedance matching and line termination. And 3 galvanic isolation options (transformer, optocoupler, capacitor) all have very different impedance characteristics. For optocoupler that impedance is moreover voltage-dependent - i.e. that part is non-linear. Yikes. Say hello to harmonics.
Peer comment explained why this is almost certainly a bad choice - not to mention you'd need some pretty specialized optocouplers to even operate at that high a frequency - so I suggest getting a receptacle with integrated magnetics. Ideally on a nice little breakout board like this SparkFun MagJack breakout board. [1]
Plus this way you can hook up a couple extra GPIOs and make the lights blink when the magic is happening.
Yeah. Modern (but not bleeding edge) optocouplers can do 100ns pulses which would give you 10mbps - but I’m not sure how that plays with the failure mode for the transmissions. Ethernet doesn’t care how many packets your sending per second, it’ll try to do the full speed from the get-go, so the question is whether the errors would be exacerbated or alleviated by retransmission from higher up in the stack.
Keep in mind though, that even 10BASE-T works with high enough of a frequency, that you need to think about impedance matching and line termination. And 3 galvanic isolation options (transformer, optocoupler, capacitor) all have very different impedance characteristics. For optocoupler that impedance is moreover voltage-dependent - i.e. that part is non-linear. Yikes. Say hello to harmonics.