As Nate explainded, I just bound a Global event to refresh the grid by adding these lines to lib/pkp/js/controllers/grid/users/stageParticipant/StageParticipantGridHandler.js:
// Refresh this grid upon submission metadata update
this.bindGlobal('ul.submissionUpdated', function(data) {
this.refreshGridHandler();
});
I guess you can set a Global event in the controller when unassigning a reviewer like this:
// Pass a global event to indicate a reviewer was unassigned
$json->setGlobalEvent('reviewerUnassigned', array(
'submissions' => array($submission->getId()),
));
And then catch it with bindGlobal, but you’ll need to find where exactly to set the event and where to bind it.