Running a process at the interaction vertex of any *other* process
One feature that would be extremely helpful for radio (and potentially other processes too) is be able to create a process whose interface is called at the location of any other interaction from any other process. This "watcher" or "vertex" process would not limit the step length or participate in the shower development, but every time an interaction occurred from another physics process, this "watcher" processes is called with the same arguments so it can "do something" with (but not modify) the interaction vertex.
This is useful for the "endpoint" algorithm for radio emission that is evaluated (almost exclusively) at particle interaction vertices.
I don't think this is currently possible but maybe someone more familiar with the cascade implementation could confirm.