My apologies if this has been addressed. It seems like a basic point that would have come up before; however, I was unable to find it on any of the other POE extender discussions.

In general, extenders provide the extra length for the segment out from the extender, not to the switch.  So in your case, putting the extender at a midpoint vs. near the switch is likely not an issue.

Which extender were you looking to use?  Using something like a Veracity Longspan requires installing one converter at each end, but the 500 feet run between is within spec:

The issue I see long term is if you get a lift and put the extender half way now is that when this device fails, and it will, you are going to need the lift again. Brian's suggestion above would eliminate having to install an extender up in the ceiling where you would need a lift. The one time lift cost could be more than using a media converter like the one above. Essentially you would use the twisted pair converter to get your Ethernet extended to the length you have. Also 500' is no problem with most media converters these days, even pushing POE across.

Use the type where one goes on the switch end and one on the camera end. If you mentioned it is pain getting to the midpoint today service and troubleshooting in the future will be even more difficult. 

Does ths longspan work if you are using coax and an encoder at camera / NVR end?

Some extenders are really more like a 2 port switch, they are regenerating the signal, keeping everything "real" Ethernet, and then giving you another 100 meters of segment length. For this type you would need to be within 100M of the end segment.

Other extenders convert the Ethernet signal to some other format, generally an xDSL-ish signal, which can cover longer distances. With these you can often go 1000ft or more, but either way you only need to be within their rated spec of the end point, and there is generally not much benefit to putting them any closer together than the rated spec distance.

Overall, I tend to not prefer little islands of equipment in the middle of a cable run. It's often more hassle to install, and makes it harder to do a proper cable test if you suspect some other activity in the building may have damaged the cable.

In your case, I'd look for an Ethernet extender rated for 500ft or more and put the devices at the far end of the cable and leave the overall cable run in tact.

We have successfully used Altronix PACE modules as “end-span” POE extenders for up to 1000ft. They work very well. You need a transmitter and separate receiver but specifically you’ll want the  “mini transceiver” for mounting out at or near the camera; as it will be much easier to fit in a box. They sell a variety of kits or items individually. 

Here is cut cut sheet on mini transceiver  it will work with any single or multi port receiver.

Altronix IP and PoE+ over Extended Distance CAT5e
Hardened Mini Transceiver

Or... just hook it up and see what happens.  
I did that with a run of about 450' of cat 5e with no issues.  I put my network tester on it running at 100Mbps and a couple tests came back just over the threshold but a camera running at 4Mbps will not hit those marks.
A year later and all is well.

What type of cable are you using?

Take a look at this company, they have solutions that will span 1,100 feet.

One main point to consider is environmental factors. If these will be outside make sure it meets the conditions of the area.