The PSE decides on which pairs power is supplied. A compliant PD needs to be able to use either pairs.
Connecting a power supply to the spare pairs (only possible with 10BASE-T and 100BASE-TX) is not compatible with IEEE PoE. While most devices would probably work, some won't, and some may even break.
My advice is to use a proper PoE injector and modify it so you can lower the output voltage. While you're at it, get a 1000BASE-T compatible one.
The easiest way would be to disconnect one of the middle contacts from the PD-side magnetics and reconnect with a variable resistor in between (50 Ohm or so) - make sure the poti can handle at least 5 W, better 10 W. You'll likely have to turn the resistor all the way down at first to make the device MDI signature work.
(Active) PoE requires the magnetics/transformers to have center contacts where you can inject or extract power without messing with the data signal which is essentially floating. If you add an in-line resistor on the center contact, you divide the PSE voltage between the PD (plus cable) and the resistor. Put another way, the resistor limits the current which causes the PD voltage to drop.
Check out Figure 1.2 from here where you can see the center tap on the transformers where VA1 and VA2 are connected.
For better voltage control, you can try using a programmable low-drop voltage regulator instead of the poti - the injector might likely fail to detect the signature this way, and the setup won't work.
Details to 802.3 PoE ("DTE Power via MDI") can be found in IEEE 802.3 Clause 33.