After reading up a bit and making some measurements with a multimeter, I can give some feedback. From measuring the impedances between the pins of LAN1 of the RUT955, it looked like pins 4-5 (+) and 7-8 (-) were isolated from pins 1-2 and 3-6. Hence it seemed safe to apply a negative potential to pins 3-6 without closing the circuit with pins 4-5 (+) and frying some electronics. This was also the case - when I connected the Gigabit injector, the RUT955 powered up fine without any smoke or other problems.
However a word of caution - the impedances of the pins of LAN2 and LAN3 were a bit different. On those ports pins 4-5 are connected via 150 Ω to pins 1-2 and 3-6, which is normal for non-PoE ports. However that would mean that if you accidentally connected that kind of injector to LAN2 or LAN3 (perhaps also LAN4 / WAN), the connection between pins 4-5 (+) and pins 3-6 (-) will close the circuit, which will most likely damage the port and possibly the router. (Pins 7-8 seemed to be capacitively coupled to the other pins, offering some DC-isolation. I assume that this is to provide PoE-protection for the "normal" scenario of only using pins 4-5 (+) and 7-8 (-) for PoE.)
However use at your own risk, since your scenario might be a bit different.