Sorry I don't quite get what you're saying, if you have sfx psu in lower position, you can either plug the ac cable directly into psu or use the same ac extension cable that comes from the upper back side.
Sry, badly worded on my part. I meant to say possibility to route power cable to lower back corner bottom part where the ventilation is while using ATX PSU. As the cable lenght would be different to top one i think cut out like in top part could be sufficient.