The 3DS Capture Kits/Dev Kits have 2 large cables going from the 3DS to the kit where the game cart normally goes inside the unit (so right where the IR sensor is). There is no way they could fit the circle pad to one of those things. I work with those kits on a daily basis and while obviously super usefull they are also very clunky.
Edit: Thanks google! Here's an image:
Those two cables are such a heavy gauge that they add a lot of downward tension to the 3DS in your hands too. I figured out a way to balance it on my knee for the RE video where it stopped being an issue, but it's been kinda tough to play on in the past. Shame we can't get direct feed of the Circle Pad Pro in action, but outside of necessary coverage I'm kinda tempted to forget that thing exists.