Is there, or might it be possible to create, a keyboard shortcut to run an assigning expression and print the result, as when wrapped in parentheses?
There isn't one, but you could make one without too much trouble. Check out https://rstudio.github.io/rstudioaddins/; the API has methods to get the current line of code and/or selection, and to send any text to the console, which you could combine to do what you want.