Hanging meat really isn't that risky if done properly, and plenty of commercially available meat is in fact aged via hanging (ever seen Rocky?). The key is temperature and ventilation. Meat hung outdoors shouldn't have much issue with the later, but is at risk in the former case. Temperature impacts bacterial growth in distinctly non-linear ways. Ideally meat will be hung at or even slightly below typical refrigerator temperatures (~37-40 F), and even brief spikes of the temp above 50 F represent a serious risk. In the UK, winter temps that remain below 45 F for prolonged periods are common, but Japanese winters are slightly warmer and temperatures would be significantly more likely to breach the critical 50 F threshold during the day (especially in the lowlands, this is not an issue at altitude and winter hunting is a feature of the Matagi culture).