The only Power Supply Rail with any headroom is the 12 VDC unregulated.
Pin 2 of J117 is spare and should be the 12 VDC unregulated.
If you are using a Color DMD this Rail will already be loaded more than the Stock Game.
If BR5 on the Driver Board is not too hot to touch then you are go to go.
If you are using a Rottendog Driver Board then it will be fine. They used Discreet Diodes for the Bridge and they take handle more current overall
As mentioned you can use 5 VDC Led Strip and splice off the exisiting Rear GI String. If you are using incandesant Bulbs in the GI lighting then you will notice the LEDS flicker during dim Illumination. So it depends on what has already been done which direction you go