We talked about this a while ago, I finally went and tested. On the moving plat on SV (and probably the parts of RC where the stage moves horizontally), we can infinite grab release MK, lucas, (I assume, didn't test) Pika, as long as you are releasing them and traveling the same direction. When you are about to get to the end and the plat wants to go back the other way you can pivot grab. The plat pauses for a while before going back the other way however, so if an opponent is mashing they will probably break out before the plat continues back.
This also works vs various other characters if we grab them at the edge of a plat to force an air release.
Now what's interesting is that we can also land a dsmash if we air release like this. The unfortunate thing is if you dsmash and do a standing grab a character like MK will never touch the ground, so if you try and release to dsmash again he will take damage but will not be restunned. If you pivot grab they do touch the ground (or atleast I know this to be true with Wario, this is why it was originally thought that we had a dsmash pgrab infinite).
Here are my random thoughts on this. Thinking in terms of the SV plat, the main problem is that like I said before it stops for a while before going back the other way, so even though we can pivot grab they can still mash out before the plat starts moving again.
If you dsmash as the plat is coming to a halt, then pivot grab just as the stun wears off, you may be able to take enough time to get around the plat's transition time. This also may allow us to bypass the one stun til they touch the ground aspect of dsmash and use dsmash pgrab each time the plat is in it's transition period.
What this all means is that, we -may- (big ifs in this scenario) have a true infinite on any character we can air release CG if we release them on the SV plat, and possibly other stages for periods of time (RC)