Using the cable to distribute power to multiple attached devices is common, but in such a case I still recommend to use DC-DC converters.
* One reason is you can use a higher bus cable, which lowers current and voltage drop across the cable. ( 1A is sort of maximum for std. Ethernet cable, less is safer)
* Power loss / heating / fire hazard is less with low current limited power supply on the cable.
* The DC-DC compensate for the voltage drop over the cable. (With 5V over the cable you have very small margins).
* Long cables are antenna's, which both receive and transmit. the Inductor in a DC-DC converter filters that noise significantly. Without a DC-DC converter you will need more filtering. Just adding more (ceramic) capacitors does not help much. You need inductors for good filtering.
When there is a potential for high DC voltages over a cable, then protection of the electronics is a real concern. Especially for wiring faults etc.
You can use dual polarity TVS diodes to short overvoltage on the CAN bus, and also add PPTC fuses in the CAN wires to protect the TVS diodes from too much DC current. Be carefull with the PPTC fuses, these have a maximum voltage rating which can be quite low (30 Volt for some).