I tested that 5A whiff 6AA j.{C} combo on a few characters. The results have been sitting in a txt file for a while, so I figured I'd post it. I couldn't land the combo on Shiki, but Tiggy said he could. Turns out my game was set on arcade balance. After switching to original balance, it worked on Shiki. I don't want to re-test everyone, so the following characters work with arcade balance, but possibly not original. Also, I didn't get to testing it on the Maids, and some others, so this isn't comprehensive.
Kouma
-mid dash 5B 5C 2C (cancel into 5A exactly as the second hit of 2C lands)
-corner IAD j.C, 2A 5B 2C
-corner (dash) 2A 5B 2C
Ciel
-mid dash 5B 5C 2C
-mid dash 2AB 5C 2C
-corner dash 2AB 5C 2C
-corner IAD j.C, 2B 2C
Warachia
-mid (dash) 2A 5B 2B 2C
-corner IAD j.C 2B 2C
-corner IAD j,C, 2AA 5B 2C
Roa easy
-mid 2A 5B 2B 2C
-mid IAD j.C, 2A 2B 2C
-corner 2A 5B 2C
-corner IAD j.C, 2A 5B 2C
Arc
-mid (dash) 5B 5C slight delay 2C
-corner no, but dash 5B 2B 2C is so close
Warc
-mid IAD j.C 5B 5C 2C
-corner dash 2A 5B 2B 2C
V Akiha
-corner (dash) 2A 5B 5C 2C
-corner IAD j.C 5B 5C 2C
-mid (dash) 2A 5B 2B 2C
Mech easy
-corner (dash) 2A 5B 5C 2C
-corner IAD j.C 2A 5B 2C
-mid (dash) 2A 5B 2B 2C
-mid IAD j.C 2A 5B 2C
There are actually quite a few different opening strings and spacings that work on each character. The timing and delay between button presses is important, since, for example, delaying one or two frames between 5C and 2C is the difference between getting the optimal distance and dropping the combo.
However, on some characters, the actual difficulty in landing the combo isn't the spacing, but the timing of the 5A 6AA after 2C. Sometimes it works if you mash 6AAA after the first hit of 2C, and other times it only works if you delay slightly until the second hit of 2C. I think it has to do with the way each character's hit box changes when hit with 2C. Since it's a quick 2 hits, the character goes through two very fast hit-box animations. Depending on exactly when you cancel into 5A determines what part of the animation they're in. I don't know MB frame data well enough, but I would think it has something to do with a hit-stun style animation stop, at least on the part of the receiving character. In other words, I think that for some characters the only way to whiff 5A at the proper distance is to hit it during the first few frames (possibly the very first) of the second hit of 2C's hit-stop.
I picked H Satsuki because of the easier execution requirements relative to F or C, but I enjoy looking at combos like this and figuring out how they work. Anyways, I don't think it's practical, I've only seen it tried in one match video, I think, against Warachia, and it whiff failed.