Well, initial excitement has died down a bit. My testing is limited to the tools available in 3ds training mode (I don't have a capture card, so I can't get exact frame data), but here's what I've found:
The input buffer seems to be a little more clever than I would expect. As far as I can tell, if you buffer a jump while a jab is out, then input an attack before the jab is finished, the game will discard the jump entirely and put out the next jab. This means you cannot buffer the entire jump canceled up smash while the jab is out. This doesn't mean the technique is invalid, only that the inputs are not as simple as they could be. You can buffer the jump as soon as the jab begins to retract, then cancel the jump animation into an up smash as soon as it begins. This sounds a lot more difficult than I've actually found it to be, so I think there's more to the situation, IIRC smash 4 has an 11 frame input buffer, so maybe you just need to have a few frames separating the jump and up smash inputs to prevent the game from discarding the jump.
Again, this is just what my testing suggests to me, I'm eager to be proven wrong. Without any way to actually see the contents of the input buffer it's impossible to know for sure if my inputs are being buffered, or if my timing is off because I'm playing in training mode at 1/4 speed. When I switch to full speed, I seems like buffering the jump and up smash right after each other works, but it's very difficult to tell. Maybe someone else with more experience testing stuff like this will be able to shed some light on the subject.
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EDIT: The technical analysis in this post is based on some incorrect assumptions I made. I was trying to evaluate the behavior of the input buffer while playing at 1/4 speed in training mode, but the input buffer does not scale correctly at reduced gameplay speed. When playing at 1/4 speed, you have the same amount of time to buffer inputs before an animation ends, not the same number of frames. I expected slowing the game down by a factor of 4 would also increase the size of the input buffer by a factor of 4, but it doesn't. The jump inputs I believed were being discarded by the game engine were actually just being input too early, so I was just buffering an up smash, not a jump canceled up smash, which the game then converted into a jab.