You could probably do something like this with the maya API.
A not very efficient way to do it would be something like:
import maya.api.OpenMayaUI as omui
import maya.api.OpenMaya as om
vertPositionsX = []
vertPositionsY = []
for vert in allVerticesPositions:
#convert the vertex position to a maya MPoint object, 'cause that's what worldToView expects as input
pos = om.MPoint(vert)
#gives you the on-screen x position, y position, and whether of not the vertex is within the clipping plane
xPos, yPos, visible = omui.M3dView().active3dView().worldToView(pos)
#if the vert isn't being clipped by the camera, we'll want those positions.
if visible:
vertPositionsX.append(xPos)
vertPositionsY.append(yPos)
#get the positions we'll need to build our square (i.e. which verts are highest, lowest, farthest right and farthest left?)
top = max(vertPositionsX)
bottom = min(vertPositionsX)
right = max(vertPositionsY)
left = min(vertPositionsY)
#viewToWorld expects a dir so we'll make one, but we won't use it later.
dir = om.MVector()
#make MPoint objects for each curve point we'll need
topR = om.MPoint()
topL = om.MPoint()
botL = om.MPoint()
botR = om.MPoint()
#Feed in those top, bottom, left, and right positions we got earlier and use them to get an in-scene coordinate for each curve point
omui.M3dView().active3dView().viewToWorld(right, top, topR, dir)
omui.M3dView().active3dView().viewToWorld(left, top, topL, dir)
omui.M3dView().active3dView().viewToWorld(left, bottom, botL, dir)
omui.M3dView().active3dView().viewToWorld(right, bottom, botR, dir)
#make the curve!
mc.curve(degree=1, point=[topR, topL, botL, botR, topR])
If you do go with something like this, a good way to get all vert positions for an object can be found at this link.
You'd have to do that for every single frame for every single object you want a bounding box, so it's not ideal, but hopefully it points you in the right direction.
//edited to add code comments
