This is a reconstructed source code for the package PmxExporter version 1.2, whose author I don't know. It has been compiled and tested to work in the latest version of BepInEx.
Furthermore, I have removed some code that was causing it to conflict with other mods, namely BoneModX, because this mod used to delete all animators in the scene. It can now export custom characters using BoneModX.
Compiled binaries are available in the releases. To install, place the .dll in the BepInEx plugin folder. There will be an "export" button in the top-left corner of the in-game screen, which can be used in the character creator to export the character model at any time. However, you will have to make sure to select the T-pose before doing this for a good result.