Pin definitions for JTAG can be found in MK20 datasheet.

Based on Teensy 3.2 schematics, the pins needed for JTAG are already connected to MKL0 bootloader chip. 

This great article outlines a nice trick for using the JTAG lines, without having to physically cut the lines between teensy and a bootloader chip. Namely, there's a test point with a reset signal for for bootloader chip, preventing it from interfering with the signal lines.

Testing it all

Hooking it up to a blackmagic probe running my branch, I could confirm the approach works since the JTAG scan lists a device correctly. This simply starts GDB on a computer, connects to a blackmagic probe on a serial port and instructs the probe to scan JTAG for devices.

And a test setup