0-10V on RJ45 jack - bad practice?
General rule of thumb is that 0-10V signals stay inside the electrical cabinet, and only 4-20mA signals leave the cabinet.
This ensures signal integrity, since a low impedance current loop is more robust, and you know when there is no recipient anymore due to cable loss or unplugging. Since the minimum of 4 mA is not reached.
Although it seems you are stuck with a 0-10V input. This is probably so users can wire up a potentiometer to easily control the speed. 

Power over Ethernet is designed to be compatible. This is due to Ethernet using transformers, and POE only using multiple pairs to transfer power, if you stay within a pair you'll not receive more than only the ethernet signals. But you need 4 wires, which is two pairs. Meaning you could receive PoE and fry it.



If you are afraid of user mistaking the 8P6C of signals vs ethernet then maybe don't use 8P6C at all. Or at least not for anything that isn't ethernet or other a balanced differential signalling.
Instead use RJ12 (6P6C) or DB9, or a plain terminal block.

You can get DB9 so it is still possible to wire them on a standard UTP cable by a terminal block in the connector. For example Phoenix Contact SUBCON-PLUS-M/AX 9 - 2904467. Though expensive, I'm sure there are cheaper brands available locally.