Is the LilyPad Arduino Main Board physically capable of interfacing with, let's say, a LeddarTech LeddarOne Optical Rangefinder, which has a 3.3v UART interface? I understand that the LilyPad has a serial interface, but I wouldn't be sure if it was meant to handle more than just transferring code.
Yes you can.
Lilypad <> Other device using UART
Rx < Tx
Tx > Rx
Note that you cannot use 5V with the lilypad if you do this. If you do want to use 5V with the Arduino, then you'll have to put a voltage divider between the Tx and Rx (other wise you might fry the other UART device using 3.3V.
Also, when you upload the code FROM your computer TO your Lilypad, you have to connect all other UART device connected to the Lilypad. Connecting other devices to the Tx and Rx of the Lilypad while uploading a code to it might cause lag or bugs.
Hope I anwsered your question!