I think this is a cool idea, and would love to see it implemented, especially in the Modern Designer.

In the meantime, we can do this via Javascript in the Classic Designer: 

$(document).ready(function () {
  //Update the title of the submit button - similar to helptext.
  $(".action-btn.Submit").attr('title', 'Press Ctrl+S to Submit.');
});

//Listen for Ctrl+S keypress and click the Submit button when detected.
document.addEventListener('keydown', e => {
  if (e.ctrlKey && e.key === 's') {
    //Prevent the browser Save dialog from opening.
    e.preventDefault();
    //Click the submit button on the form.
    $(".action-btn.Submit").trigger('click');
  }
});