Module requires Composer install of third-party library
For anyone else facing a similar problem, I just wanted to add that, as of Aegir 3.18, it seems possible to configure Composer to download and install third party libraries for Drupal modules from https://asset-packagist.org. Of course, those libraries should exist as Composer packages on the asset-packagist site.

I was able to get this going yesterday by following the instructions at the link below, but after a lot of trial and error as the docs are not always accurate, especially in terms of the line endings, JSON syntax etc.:
https://www.drupal.org/docs/develop/using-composer/using-composer-to-install-drupal-and-manage-dependencies#third-party-libraries

The composer.json file to edit is at the Aegir path /platforms/drupal-x.x.x/composer.json