It has been made possible to exchange foreign currency at Snow Can, the rental shop for ski/snowboard wear and more in Shiga Kogen. They have several shops, but the only one providing exchange of currency is the one at the Ichi-no-se Ski Area.
The machine can be used between the hours of 10:00 and 16:00.

The currency accepted at Snow Can is:

EUR, CHY, USD, HKD, TWD, KRW, THB, SGD, AUD.

So for those still having some of their own money, will be able to get a bit more cash if needed.

Caution: not all currencies are available.