I'm by no means an expert on this but it appears that you can configure the preseed to add late_commands. For example, you can configure the preseed to install a package based on the hostname, and after the installation has been completed:

late_commands:

{{if node.hostname == 'node01'}}

    package_install: ["curtin", "in-target", "--", "apt-get", "-y", "install", "mysql"]

{{endif}}

Note the following:

Warning

Do not try to alter the preseed files if you don’t have a good understanding of what you are doing. Altering the installed version of Ubuntu can prevent MAAS from working as intended, and may have security and stability consequences.

Source: https://maas.ubuntu.com/docs/configure.html#altering-the-preseed-file

Further reading:

http://maas.io/docs/en/installconfig-images-mirror

http://maas.io/docs/en/manage-cli-images#select-images