Put it on the BACK layer.
You'll need to position it carefully because the anatomical reference for the BACK doesn't actually
reach the breasts. Depending on which template you're using, you might find it useful to copy the adjacent bodyparts (as Guide layers) onto your canvas.
Remember that the breasts will continue to exist 'underneath" the breastplate. Once you've chosen a preferred size, remember to leave some "wiggle room" so that the breasts won't clip through the breastplate during bouncing or heavy breathing. If you want the breastplate to fit tightly, then you can draw it at the preferred breast-size and shrink the
actual breasts to minimum (so that they won't poke out). It gets more complex if you actually
want to show off some cleavage because then you'd need to keep the breasts at their full size. In that case, it might be necessary to apply a mod which temporarily
suppresses breast-bounce while your breastplate is active.
The BACK layer has visual priority over the CHEST layer (and related layers: UNDERBOOB, CHESTUNDER, etc). Therefore you can still draw form-fitting stuff (such as chainmail or regular clothing) on the CHEST. The form-fitting stuff will move and stretch as the character breathes, while the breastplate will remain totally rigid.
As
Huitznahua
mentioned, there are also more complicated options available if you're willing to incorporate physics into your clothing. Such features are useful if your design includes rigid elements
but you still want to support the breast slider -- the rigid elements will
move as the breasts are resized (or bounced) but they won't actually
stretch (because they maintain their rigidity). Unfortunately, such physics features depend on the Loader ... so they're probably not available in this case :(