Don't know if this little script helps, but it will give you a custom menu command which will select all images that have an associated mask.
From there you could use the inbuilt commands to filter by selected images or move those images to a new group/chunk.
import PhotoScan
def selectMasked():
app = PhotoScan.app
chunk = app.document.chunk
for cam in chunk.cameras:
cam.selected = cam.mask is not None
PhotoScan.app.addMenuItem("Custom Functions/Select Masked Images", selectMasked)