Bonus question - Probably anything is possible, but without a whole lot of work... It is pretty well fixed to the menu. But you can go in and make a custom version, by editing the core code to define all of the descriptors and the like.
MTP Plus Serial - Right now the builds are setup that they support the Serial Emulation code base. That is if you connect up to the Arduino Monitor (Teensy version), you can connect up and do Serial inputs and outputs. Also I have put code into USBHost_t36 library (merged within the last few days) that has SEREMU support and likewise TyCommander has it as well.
But @WMXZ has some setup for MTP plus Serial which is not integrated into the builds. It uses one of the other Predefined Product IDS for Teensy. But in the MTPD directory there are directories, with files that you can use to update the boards.txt to add that option, plus files for T3.x and T4.x that update the usb descriptor header file for these options.