If you don't need compatibility with ethernet, it doesn't matter which pair you use. It doesn't even really matter much that you use pairs.
Your main concern in this endeavor will be the resistance of the cables. You can find a chart of wire resistance and from that, calculate the total resistance of your cable. For an example, cat-5 cable is typically 24-26 AWG. From a table, I see that 100 feet of 23 AWG wire has a resistance of about \$3.2\Omega\$.
Say you want to move 500mA on this wire. We can calculate the power lost in the cable:
\$ P = I^2 R = (0.5A)^2 3.2\Omega = 0.8 W \$
or the voltage you will lose on this cable:
\$ V = IR = 0.5A \cdot 3.2\Omega = 1.6 V \$
Keep in mind, this is for 100 feet of wire, but since you need a supply and a return, the total length of the wire is twice the length of the cable.
This 0.8W is not only power not doing useful work, and less power available at your load, but also heat generated in the cable. A longer cable has more area to radiate heat, so can dissapate more power. This is why most tables also include a maximum current. \$500mA\$ is just a bit too much for 23 AWG cable.
Also, the voltage drop will reduce the ability of a voltage regulator to regulate the voltage on the other end. The voltage regulator has no way to know what the voltage on the other end is, so it can't regulate it.
Another concern is the maximum voltage the insulators in the cable can withstand. Power over ethernet provides 48V, so anything below this should be safe. Anything above that, consult the datasheet for your cable.
Firstly you will need to check your end devices are 802.3af mode b compliant, otherwise you can't inject the power onto a discrete pair, but instead if they are mode a, you need to put the power onto the data pair which is usually a function of the switch.
With regards to UTP. It is possible you might be able to use non twisted cables if the distance is short, your devices will put up with a lot of dropped packets, sub 10Mbit speeds are ok and you don't have too much issue with external EMF sources.
But I would be much more inclined to simply run two shielded UTP cables.
Best Answer
I used to run USB and Ethernet in the same cable, to connect a router to a printer. I made two custom adapters to inject the 4 USB signals to the unused 2 pairs in 100Mb Ethernet. It worked very well, until an electric storm fried the USB port of the router; I don't know if the setup contributed to the failure. If you try it, make sure the USB data lines use a twisted pair, and 5V+GND use the other. In my case the distance was about 4 meters.
Edit about the failure: There was also a telephone line running next to the USB+Ethernet cable in the last 2 meters. Since the electrical storm also fried the ADSL modem, I think a voltage spike in the telephone line coupled into the USB wires and fried the port on the router.