https://www.amazon.com/Splitter-Compliant-Surveillance-5-5x2...?

You can also find variants of them that are microUSB or USB-C power out. Finding ones that are natively 3.3V or 5VDC out is more difficult and rare, it seems the most viable option if I do this is to buy a very tiny DC-DC converter that's 12VDC in and then 3.3 or 5VDC out, and glue/stick it to the poe splitter, perhaps in an enclosure of some sort. Then wires out to the battery dummy eliminator.

The thing above is a 'dumb' device so any monitoring of the power you would have to do from your 802.3af PoE switch, entirely depending on what model/capabilities your switch has for port status, wattage, etc.

edit: Google "USB AA battery eliminator" and you'll find products which have a USB-A male input, expect standard 5V USB input, and output either 1.5V, 3.0V or 4.5V or 6V (1S, 2S or 3S or 4S AA battery voltage). You could then plug one of those into the many 802.3af to USB-A female (5V2A) poe splitters which exist on the market. Example: https://www.amazon.com/Battery-Eliminator-Supply-Adapter-Rep...