Can one purchase from you the MK20DX256VLH7 microprocessor used in the Teensy3.2 (or any of the other MPs) programed with the arduino bootloader to use in my own designs?
All of the bootloader code is stored in the MKL02 chip, so you buy that chip from PJRC and pair it with a blank MK20DX256VLH7, which you can buy from Digikey, Mouser, Avnet, Arrow or any other distributor that sells it.
If doing a custom design, see notes at the bottom of the thread and more recent forum threads on some of the gotchas is this sort of thing.
And yes, if you are really confident that your design will never need new firmware it is possible to put a bootloader on some sort of programing module and just use it to program the entire production run.