Locked bootloaders can be a real drag, but there's usually some way around it. In the case of the Verizon and AT&T Galaxy S4, Dan Rosenberg released the Loki tool to bypass the locked bootloader and execute custom kernels. This developer-focused tool has been a hit with its intended audience, and now it has added support for LG devices.
Loki can be obtained from GitHub with all the necessary code and documentation. In addition to the Samsung Galaxy S4, the app can now bypass locked bootloaders on the LG Optimus G Pro, LG Lucid 2, LG Motion, and LG Spirit. The process should be roughly the same for the LG phones as it is with the GS4.
It probably bears repeating, this is not a user-focused tool – it's aimed at ROM and recovery developers. You are, of course, free to play around with Loki, but it's not going to be as simple as flashing a ZIP file. Grab the code below if you're interested.