Yeah, knockdown is pretty hard to justify, especially if the combo doesn't grant you enough time to pull a pumpkin out safely.
Anyway, correction on the elevator combo: should be j.b j.c 2d JC j.2c.
I knew I'd gotten something wrong in my notation, but I couldn't remember what, until I went and did the combo. :/
The particular usage of the combo seen in the video is probably Tager/Hakumen and corner specific, due to the fact that they have large hitboxes and will get caught in the entirety of BBL. On other characters, a 63214d6c seems to be the necessary response, or a dash after the j.2c before BBL (which may or may not work, haven't tested; dash 6a32146c works in both CT and CS, though, so it's possible).
Edit:
Landed it midscreen on Ragna, but I keep whiffing the full BBL on Lamda, so I'm not sure who it does and doesn't hit without extra wind.
Level 2 j.2c starter gets around 4.4k. Not that it's going to happen normally.
Also, if you wanna be silly, or you can kill someone with one more hit, any lobelia hit in the air can be RC'd into sword iris or air iris. Might be combo-able into BBL on counterhit, too.
And 6a 2c combos in certain situations. :8/: Not useful, but also funny.
Edit 2:
Played with the elevator combo a bit more. It definitely works on all characters, though the timing is a bit tighter on some than on others.
It breaks down mostly into two important parts:
1) Make sure you do 5b immediately after 6a lands, or a couple characters will float too high for the combo to land.
2) When doing the j.b j.c 2d JC j.2c, the goal is to reach a level 2 j.2c height before going into j.2c. You want to wait as short a time as possible, but still get a level 2 j.2c for its ground bounce.
Also, on Tager, if you aren't confident in your execution, you can use a SJ j.b j.c 2d JC j.2c to make it a bit easier.
An alternative, easier combo for 2002 dmg (about 300 less than the above) is 6a 5b j.b j.c 2d JC j.c land SJ j.b j.c JC j.b j.c j.236a.
Both combos can combo into BBL during the landing section. The elevator combo gets around 3k damage (slightly more, iirc) off of BBL to a lobelia and frog, while the alternative gets around 2.7-2.8k off the same setup.