You can easily use either the usb or 3.5mm port to send out signals triggered on whatever event you'd like, the hard part is the power for the LEDs.
If you go usb you'll need a USB OTG cable for a start to allow sourcing voltage from the micro usb port, BUT I'm not sure about the max current you could pull. According to this doc it could be as little as 8mA http://www.nxp.com/documents/leaflet/75016113.pdf
(bottom page 8)
If so you'd only be able to power something tiny like this lil guy SMLE13EC8TT86 | ROHM SMLE13EC8TT86, SML-E1 Series Blue-Green LED, 535 nm 1608 Clear, Rectangular Lens SMD package | ROHM
But looks like it'll vary from phone to phone
Alternatively, you could power it off the 3.5mm audio jack. Output from that is ac and variable is the only catch. You'll need to convert to DC or just ignore half the cycle and throw in a capacitor, losing power and complicating your circuit either way though. Google around and you should find a couple projects that've done this already.
Alternatively alternatively, you could add a small external power source, e.g. a coin cell battery. Maybe you could build all this into a custom case? Then we got no probs
3d print a shell that you can slide together over the phone mb.
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