Yes, you do need to use a loop for your external pedal. One of the other suggestions though was to use the Distortion Loop Instead of the Mono or Stereo Loop because Distortion Loop allows control of volume.
Wah Volume is one of the Wah FX types which lets you use the Wah in the Stomp or FX blocks as a volume pedal at whatever point in the signal path you insert the Wah Volume FX. It functions exactly like the normal volume but can beused as a second volume so you can have volume pedals at more than one point in the signal path and control them independently. There is more information in the manual. Its a great idea but badly named as it is totally counnter intuitive and most people (including me) react the same way as you did when told about it - "what's wah volume?"
Ah, I see. Well, Kemper introduced me to so many types of Wah's with weird names, that it would be conceivable that's an actual type of wah
That's a useful trick, but my main goal here was to refrain from wasting two post stack effect slots, just for the looper.
I don't have an issue with using the distortion loop in itself.
This is why I had come up with the solution I mentioned earlier. By using the volume pedal as a post stack booster, and using the distortion loop at X, I can achieve a pure booster that doesn't waste a slot, and goes into the looper input. Well, almost a pure booster, because it can only boost, and not attenuate.