One way would be a PCB that mounts underneath the Teensy, connecting the 6 pins from the ethernet, 5 pins from USB host, VUSB, and VIN. These would be soldered directly to the Teensy with normal male pins. They bring out to one of the sides the pins, and you have mounting holes for the combo board you are using, and any necessary capacitors, resistors, etc.
I could imagine a 5v connection (either 2 2.54mm pins, a USB micro-B header, or a 5.5mm x 2.1mm power jack), and two switches. One switch is between VUSB and the external power connection and it connects to VIN. The other would be between VIN and the external power connection and it connects to the USB Host power pin. That way you can switch to use VUSB to power the Teensy, but external power for USB host. Or you can power both with VUSB. Or you can power both with external power.
With stacking headers, the stacking headers are tall enough that it would not interfere with the normal USB micro-b connection, and you could just have the ethernet/usb directly on top. But that can be problematical if you want something like the audio shield mounted on top of the Teensy.