Since you posted this, I've done a fair amount of labbing and it's actually kinda awesome.
Turns out you don't need to do this out of a RAR aerial, but any move that forces Ryu or Ken to look away from the opponent can set up the slide. For example, if you hold down+back and press A they'll do a light or heavy dtilt backwards, which can start the slide.
The key to the slide is inputting focus on the 1st frame of his turnaround animation, but if you buffer focus for the first active frame before you turn, you'll still do focus backwards, so you need to wait 1 frame after the FAF to start the down b. Moreover, it's easier to input down+forward and press B to get the slide, or down+back. Much more consistent than sliding the stick from down to forward.
This means we can basically do the slide in neutral, with tons of moves. The FAF changes with each move, and you need to be frame-perfect, so you might need to practice multiple types. Light dtilt seems to be the best grounded option, if you do a backwards light dtilt you can start sliding on frame 17 after the start of the move.
You can also do it with backwards utilts, ftilts and fsmash, as well as any landing aerial (SH instant RAR nair works pretty well, but any crossup aerial will do if you're not good at RAR), as long as it forces the turnaround animation and your timing is good. You also have all the regular focus options as you slide, eg. Slide forward, release backward, or slide forward, FADC backwards, etc. Nekuodah has a decent vid on YouTube that shows the grounded versions.
It's a frame-perfect input to get it started, but you can use it a ton of ways. Hopefully it doesn't get patched, or if it does we at least get our nair back