Better late than never?
I considered using the internal power supply, but there are enough posts of issues with electronic oddities in the controller made me leery of tapping into it. The printer power switch cuts the AC into the unit, so I have no hesitation using an external power switch. I use a separate supply for the lights and plug it, the printer and computer into a power strip and use the switch on the power strip. Computer is set to start on AC recovery, so the whole system is controlled by one switch. I used 12v adhesive LED strips and have an inline dimmer, the strips are slid into the bottom T slot of the top rails. If you were really motivated to use the internal supply, you could wire them in series; I tested that and it works.
A forum dedicated to the stock LulzBot TAZ 3D Printer