Powering multiple devices from a single supply with 48V
Good ideas.  

Remember to have rational size fuses at the supply end of your power cables to protect against fire due to shorts in the walls. Use the thickest wire for the power pair that you can afford and draw through the walls with ease to minimise losses. 

You can add a 48->5V converter for your RPi and perhaps a higher power 48->12V converter for your IR illumination if suitable.  Such converters are commodity items.

You could use a second Cat6 cable for the power and use two pairs for ground and two pairs for +48V but it will still be less copper than a dedicated pair of the same overall size.

EDIT:
As clarified by Golanz in the comments above the voltage converters should be of the switching type and not simple linear regulators so the current in the 48V link is reduced, a linear regulator would dissipate lots of energy as heat in the regulator and cause totally unnecessary load on the power supply and wiring by the same amount. Finding linear regulators with a 48V input for 5V output would be harder these days anyway and only appropriate for rare analogue applications.