Your choice of coconut rum will significantly impact this riff on a classic Negroni; many so-called "coconut rums" are liqueurs or verging on being liqueurs...
Pretty good with Bacardi Coconut (35%). A good balance of bitter and sweet, with coconut, orange, and vermouth creating an interesting mix of flavors. Seems like a good option for the "Summer" category here on DG.
Peter McCarthy