Sluox points out that it depends what sort of informatics fellowship you do. Informatics relevant to pathology (mostly lab related) has no relevance to most other clinical specialties. Other informatics fellowships tend to be systems/public health focused and more broadly applicable. At least two of the fellowships in Boston accept psychiatrists.
I'm not sure that I agree that doing a T32 is necessarily better than a formal fellowship. You have to do a fellowship if you want to claim the clinical specialty (can't get grandfathered in at this point). It will probably help to do a fellowship or master's if you want to be a CMIO, as well. If you want to do informatics research, productivity is probably more important than credential.
Also, my own cursory review of CI programs shows most of them saying something similar to "Applicants must be ABMS board certified or board eligible in a primary specialty" which means "literally any specialty training."