Are you even using PoE devices at all? If you go to a store like BestBuy and try buying a small ethernet switch, you won't get anything managed that runs off 12v power at all. All you will get is the ones that have half PoE and half regular ports. The stores do this because they want to have a once-size-fits-all switch they don't what to stock 2 separate managed switches, a poe one and a non-poe one. But Netgear sells plenty of small managed non-PoE switches that run off 12vdc
if you ARE supplying PoE to a device, consider if you can just power the device from a separate adapter. For example I have a desk phone here that runs off either PoE or it runs off 5v DC from an adapter.
In general it's a lot easier to buck DC than boost it. If your world is mobile and you are running 12V DC power then you can feed a 5vdc device easily with just a single 20 cent 7805. But if you try to boost it, the boost device has to convert the DC into AC, then transform that and then convert it back to DC.
If you are in a telco closet you have -48v DC. That's also simple to drop to 12v and it will also directly feed most PoE injectors. Consider unplugging your PoE devices from the switch and plugging in a non-PoE switch then plugging in PoE injectors.
