What are your cites?

And is 1231 gain, such as from sale of a rental property, distinguished from capital gain?

And is this a case where you have to manually adjust the investment income for the rental property gain for the 4952 calculation?

Anything is possible, the question is whether it's legal, and maybe whether IRS even knows or cares. I see what you mean, the instructions are ambiguous.  And I'm not going to wade through the Code unless someone pays me to do it.  But to play devil's IRS advocate here, why should it be?  The interest to carry the rental property is deducted on Schedule E.  The investment interest was paid to carry other investments, not those.