Bypassing the ARINC 429 is possible by direct wiring to the ECP input (switches) and output.

New style ECP (LED output) requires positive +5 VDC on pinout 54, and shared grounding with pin 1, e.g. For Engine LED output: 54 (+5 VDC), 20 (Eng) and 1 (ground).  Short pulses are used for the input and may not be detectable on a multimeter. 

Thank you to Simon and Florian for the pinout list.

Pinouts

-BACK LIGHT 7-8

                                   SWITCH INPUT      OUTPUT
 -EMER CANC                        21-22, 1-23          /
 -T.O CONFIG                       21-24, 1-25          /
 -ENG                              21-26, 1-27       54-20
 -BLEED                            21-28, 1-29       54-19
 -PRESS                            21-30, 1-31       54-18
 -ELEC                             21-32, 1-33       54-17
 -HYD                              21-34, 1-35       54-16
 -FUEL                             21-36, 1-37       54-15
 -ALL                              21-38, 1-39         /
 -F/CLT                            21-40, 1-41       54-14
 -WHEEL                            21-42, 1-43       54-13
 -DOOR                             21-44, 1-45       54-12
 -COND                             21-46, 1-47       54-11
 -APU                              21-48, 1-49       54-10
 -STS                              21-50, 1-51       54-4
 -RCL                              21-52, 1-53         /
 -CLR                              21-5,   1-6       54-9
 -CLR                              21-3,   1-3       54-2

Pinout schematic diagram property of Simon Stusek and not to be used without his permission.

ECAM_panel_pin-out