"My plan would be to add on a 12v rail that comes out of T1 pins 9 and 10."
There's already a LED there, or is that the one you've added.
SuperBright LEDs are bright enough as an indicator at 2 or 3 mA, so you can connect them almost anywhere there's DC.
"Add a couple resistor as a voltage divider"
A LED only needs one series current limiting resistor.
The polarity of C104, C105 and D107 look the wrong way around to me.