The OP here. Great suggestions. However, my concern is not simply how
to make the cat hear me. It is doing so without making a nuisance of
myself all over the neighborhood. Maybe I am self-conscious, but I
would like to be as discreet towards other humans as possible.

I have a high pitched dog whistle I bought a few years ago in a futile
attempt to eliminate pigeons from my balcony. When I blow it, my cat gets
irritated so I know she can hear it. But, getting a cat to respond to one
might be very problematic unless you can associate the high pitched noise
with the knowledge that there's food or some special treat ready and
waiting. The only thing I can think of is to blow it absolutely every time
just before you put out some food or treats. The hard part will be
remembering to blow it unless you attach it to wherever you keep the cat
food.