With help from [https://www.mathworks.com/matlabcentral/answers/356708-how-to-position-a-annotation-in-subplot-southeast], I supplied "get(gca,'Position')" as the "dim" name/value parameter:
annotation('textbox',get(gca,'Position'),String="The quick brown fox",FitBoxToText='off')
Unfortunately, the text box's left edge overlaps with the y-axis labels!
A work-around that doesn't involve manual customization of the "Position" based on Figure size and number of subplots would be welcome. Thanks!
P.S. I currently finagle the position by manually resizing the text box, issuing "get(gco,'Position'), then editing the "annotation" command to use this position. Ideally this would not be necessary.