I feel like you're opening the world up to a whole host of inter-dimensional boob physics related questions and I for one would like some God-damned answers.
Assuming they use "realistic" portal physics, the boobs would remain mostly stationary in the void, since the body wouldn't be moving much relative to the portal. Also, depending on what else is in the void, you wouldn't have to worry about gravity pulling them down either.
I think there's no gravity and no air, do they would float around some. They would also be weightless I assume. I don't know how this works with things partially inside a bag of holding. I assume the forces would transfer to either side, but one side doesn't feel gravity. Maybe it'd be best to wear a bra under to keep things from floating around too much?
I'm not trans but I hate having to wear bras to keep my nipples from showing through my shirt, so I wish there was a nipple pasty variety of this in real life (as well as the binder version as well for trans peeps of course).
I mean, if it works like the bag they'd be protected by walls of force (iirc) around them on the astral plane. So its nice to know they're not subject to being eaten by an Astral Dreadnought or something.
Although that makes my DM brain consider the concept for a Berbalang cabal seeking to feed themselves on the astral plane through the subjugation/encouragement and selective attachment of cursed "holding" items (that don't have that field) onto their preferred prey.