Create a hook to set the destination path after authentication.

Thanks @msti for answering. Your patch no longer applies to the 3.2 branch; plus i don't think they handle redirecting the same way now (although don't see any way of altering this either).

I'm tempted to simply add this line in OpenIDConnectRedirectController.php

    // @see \Drupal\openid_connect\OpenIDConnectSessionInterface::saveDestination()
    $session = $this->session->retrieveDestination();
    $destination = $session['destination'] ?: $this->configFactory->get('openid_connect.settings')->get('redirect_login');

    // Add Alter to allow modules modifying the destination.
    $destination = $this->moduleHandler
      ->invokeAll('openid_connect_login_redirect', [$destination])[0];

but suspect some method already exists to do this?

no - that would be nice :-)