I've done this to run two cameras over one cable, but without using any fancy splitters - I just split out the pairs into two separate RJ-45 plugs at the switch (orange/green on 1,2,3,6 for one cam, brown/blue at 1,2,3,6 for the other).
Since I was adding a camera near an existing camera with no option to pull a new run, I re-terminated the run by the camera with the original two pairs going into an RJ-45 plug, and the other two pairs split out to a keystone, then ran a patch cable to the new camera.
One important thing to note: this will only work if your switch or injector supports Mode A (aka endspan), which provides "phantom" power on the data pairs. It won't work if the switch or injector only does Mode B/midspan, which puts the power on the two normally-unused pairs.
If you have a gigabit switch, you're pretty much guaranteed to have Mode A support... a cheap 10/100 switch may only do Mode B.
Norris, Inc., S. Portland, ME
In this day and age, everything we do is later certified. You can buy Poe plus enclosures, which are only a little bit more. Certified jack on interior of building. Patch cable through wall to heated enclosure (-40 c needed in maine) Poe electronics, which has poe interface to the camera. We now use mostly Poe plus heated domes (stopped adding the 24 vac wiring two years ago) and Poe plus or high poe ptz domes on certified cat6 drops. There is no way most customers would allow a network connection connecting to the switch and network if they knew it was modified to carry 24vac. This must be punched down on a patch panel or keystone. Seems like the risk is too much compared to paying an extra 50 bucks for a Poe plus heated enclosure.
Using low voltage tactics like this does not work anymore on high performing Ethernet networks, especially if it is a customer data network.
Norris, Inc., S. Portland, ME
I used to be an advocate for enclousures with box cameras and professional lenses. Sometiimes you need this for telephoto lenses. Now I use all outdoor fixxed domes (poe+ -40 C) with auto zoom, auto focus and POE+ power. New switches coming out are POE+, so at this point, using POE+ injectors. For new installs, we make sure the switch has at least some POE+ capability. New PTZ HD 20X cameras rated to -40C with POE+/Hi POE (and < $1000 our cost) so this is becoming an option over the tele lenses and box cameras. The case for the box enclosures was precise focus and alignment. Now we can auto focus.
We have had some problems with switch/class negotiation from POE enclosures (remember, there are electronics in their that must negotiate for power with switch). If you consider the few that we have had installed, this is not a good percentage. As far as 15 watts, my problem is that for low temperature ratings, I want watts (direct conversion to heat), so a box enclosure with all of the space inside of it, exterior surface area, and lack of insulating R value, I have doubts that < 10 watts can do much good, except for defrosting of the glass.