One thing you can do, though, which has been done before, is to create your entire design inside a custom padframe that fits inside the user area. There is no requirement to connect to the user project wrapper pins. That limits your total area considerably, but gives you complete freedom to do whatever you want with the area. Note, however, that you would need to do this on a ChipIgnite run, because all Open MPW runs are post-processed with bump bonding on top, which would interfere with any custom pads.