Hi
As far as I can see, the blueprint would be to map external house banks in Cash management module as House banks, which will also be able to perform Cash Concentration functionality (FF73). This can also create Payment requests in FI if you would like the system to instruct banks to sweep the surplus funds. In this design, JPY bank accounts would be maintained as house bank in the company code that IHC belongs to.
IHC serves as a Internal banks within the group and will be able to manage Internal payment/central payment/payment on behalf from subsidiaries. Also it has the account balancing functionality to charge Interest against subsidiaries. One IHC current account can be set up per one currency, so you could look at having more than one current accounts per currency for the subsidiary in order to calculate the Interest charge correctly.
Forex gain/loss may be able to pass to subsidiaries via IHC by provisional/final posting functionality in IHC for external payments. You would need to assign different exchange rate type to both types of payment transaction within IHC
Hope that this helps a bit.
Regards
Taro