OmniGraffle > Help > Send Feedback ... is the only channel certain to be read. Their hands are fairly full with two platforms and so many apps.

On the other hand, things sent through that channel can tend to be "written up" and then spend a number of years in database incubation [asymmetric hierarchical layouts with orthogonal connectors come to mind], attracting attention only if matching notes are numerous, so combining feedback email with discussion here may help, if that results in parallel emails being submitted by others.