| That is really an interesting situation, I personally have never been exposed to your specific situation. However, I think conceptually at least, absent a specific IRS code prohibiting this interest to qualify as an investment interest income, you would seem to be able to qualify "this interest income from the C corporation as an investment income" that can be offset against real estate interest expense!
Still, I would refer to your tax accountant who is preparing your C corporation tax return to ensure that the loan is properly documented and is an arms-length transaction. | 
