xfce has a feature where windows are being placed according to the pointer position at the time. It's incredibly nifty and useful, at least to me :)
muffin, a mutter fork, has implemented this as well, with
The code looks fairly similar to marco (due to muffin/mutter/metacity common inheritance) and the patch is fairly trivial (basically one function + stitching that to the code with an option).
It could probably very easily be ported to marco and I could give it a stab, although someone with more marco experience would probably tackle it in far less time than I would :)
I think it will apply cleanly except for the schema. Try it and send a pull request.
If you need help with git do let us know.
Looks like it's fixed by #219.