Assuming a Pi B+ here (might be adaptable to other models though).
I'd simply suggest to go for a doubling of the existing LEDs. Either i) simply solder a thin wire to the each cathodes of the LEDs or ii) desolder the LEDs on the PCB - and still solder the wire to the pad of the cathode. See schematics model B, Rev 2.1 page 3. The external LEDs would be wired in the same fashion. With resistors of 1k at 3.3V the max current per LED will be less than 3.3mA. Adding additional LEDs in parallel keeps the total current below 10mA (for the 3 LED case - PCB, jack, and external).
Checking the datasheet of the LAN controller LAN9512, pp. 10, 11 and 20, we find that the pins' buffer type (for nFDX_LED, nLNKA_LED, and nSPD_LED) is:
OD12 Open-drain output with 12mA sink
Well, seems that even a three LED solution (on board + jack + external) would be in spec with the LAN9512's buffer circuitry.