How to override version of drupal-library (JavaScript library) included with composer merge-plugin?
But unsuccessful in that it does not have priority over the one defined in webform:

There is a way, just define custom repository before Drupal packages repository .
Change your composer.json to
   &quot;repositories&quot;: {
        &quot;jquery.inputmask&quot;: {
            &quot;type&quot;: &quot;package&quot;,
            &quot;package&quot;: {
                &quot;name&quot;: &quot;jquery/inputmask&quot;,
                &quot;version&quot;: &quot;5.0.6-beta.45&quot;,
                &quot;type&quot;: &quot;drupal-library&quot;,
                &quot;extra&quot;: {
                    &quot;installer-name&quot;: &quot;jquery.inputmask&quot;
                },
                &quot;dist&quot;: {
                    &quot;url&quot;: &quot;https://github.com/RobinHerbots/jquery.inputmask/archive/5.0.5.zip&quot;,
                    &quot;type&quot;: &quot;zip&quot;
                },
                &quot;require&quot;: {
                    &quot;composer/installers&quot;: &quot;~1.0&quot;
                }
            }
        },
       &quot;drupal&quot;: {
            &quot;type&quot;: &quot;composer&quot;,
            &quot;url&quot;: &quot;https://packages.drupal.org/8&quot;
        }
    },