If I am understanding your question correctly, yes.
You could connect it; main switch to PLA (first half of the pair nearest the main switch) then PLA (second half of the pair nearest the camera) to PoE injector/PoE switch then to camera. This would work as the data exits the main switch, traverses the PLA pair, enters the PoE injector so now the network cable carries both data and power which then arrives at the camera.
The best way to think of a matched pair of PLA devices is that they are essentially an ethernet cable of unknown quality. Some PLA implementations work just fine, others are intermittent trouble and some flat out don't work. It largely depends on the electrical wiring being used, the amount of RF noise on those circuits and lastly the quality of the PLA units themselves. Keep in mind that PLA units can experience intermittent disruptions which can be very hard to trace down from things like refrigerators, electric clothes dryers, vacuum cleaners and most anything that can cause disruption on the electrical circuit(s) that the PLA are utilizing...your mileage may vary.
PoE injectors always have at least 2 data connections, 1 for just the data (this is where you would connect your PLA) and 1 for data+power (this is where you would connect your camera). If you mean you would need an injector with more than 1 energized output they do make such things but they are usual same number in and out, for example 8 data connections in with 8 data+power connections out which wouldn't be useful to you I think. As you are talking about PLA I suspect you would only have 1 data connection into that area so a multi-in multi-out solution would mean more PLA units. I think the better choice if you need more than 1 energized PoE out would be a small PoE switch. You could use the PLA connection to connect that small PoE switch to your main switch and have multiple energized outputs, as many as are needed but keep in mind they would all be sharing the single data connect back to your main network. Depending on the model of camera and its settings as few are 3-5 cameras or as many as 10-20 could be too many cameras on that single data connection back to your main switch.
Having passive connectors such as patch panels or wall jacks between the PoE injector and the camera is fine, but having any kind of device like a PLA or a switch or balun or such would not work.
