Are there any gamma emitting devices at hospitals that could repurposed for this? Such as those used for cancer therapy? Or imaging devices (CT scanners)?
I doubt it. Those devices are not designed to pour lots of kGy into stuff. Food irradiation typically involve some kind of lead-covered mini-tunnel with the radioactive source in the middle. Definitively not an expert, but I suspect the radioactive sources used for food irradiation to be two to four orders of magnitude stronger than what is used for therapy.
Dosage for food is like a 1000 Gy, for therapy on people its more like 1-10. A medical 60Co source could still reproduce the same effect by putting the masks right against the aperture and increasing exposure time. No idea if this would destroy viruses, though.
I have found literature on this, look for "Sterways Process". It seems that it starts to be safe virus-wise somewhere around 35kGy.