In Magic Model Analyst® (Cameo Simulation Toolkit®) (v2024x) on a SendSignalAction: Nested Ports won't appear available in the selection dialog for 'onPort' (or work for DnD) unless «InvocationOnNestedPortAction» has been applied to the SendSignalAction, any nested Ports will be grayed out (filtered out). This "filtering" make sense once you are used to the procedure, but it might cause confusion the first time you encounter it.
In this external video Semantic Zoom of Nested Ports Definition and Simulation from 2021, this restriction does not seem to be in place (see about 3:28 in). Maybe it's an older tool version, or maybe it's not made explicit in the video that «InvocationOnNestedPortAction» has in fact been applied to the SendSignalAction.