Use command line to upload program through hex file to Teensy-LC
Paul Stoffregen (Teensy creator) has released a command line version of the Teensy Loader which can be found on his website. However, this program appears to be Linux only, so unless your client has Linux, that may be a non-starter. The source code for the program is available, so you might be able to work out the logic and port it over to whatever platform you are wanting this to work on (just make sure you comply with the source code license).

All that being said, pressing the reset button is not necessary for programming. The Teensy loader should be able to trigger a reset over USB all on its own. So unless the current program on the device disables interrupts that are necessary for the USB port to function, or it was programmed with the USB port disabled (by selecting "No USB" in the USB Type menu), physically pressing the reset button isn't necessary for programming. For more details you can see this forum post.

Loading Firmware Without Pressing the Button

When programming via Arduino, it does a lot of things for you automatically. To replicate that manually, follow these steps:


Connect the Teensy to the computer via USB.
Run teensy.exe (found in C:\Program Files (x86)\Arduino\hardware\tools)
Click the button on the left to select the hex file to load.
Go ahead and leave Auto enabled (this actually doesn't matter).
Run teensy_reboot.exe (also found in C:\Program Files (x86)\Arduino\hardware\tools, this will restart the Teensy, as if you had pressed the button, and automatically start the programming process.)