SatisPress doesn't provide support for mu-plugins since they usually require manual installation and aren't managed by the WordPress update process.
However, if you can install an mu-plugin as a regular plugin in your SatisPress instance, you can force Composer to install it as an mu-plugin in your project.
As an example, the following configuration in composer.json would install SatisPress as an mu-plugin:
{
"extra": {
"installer-paths": {
"wp-content/mu-plugins/{$name}/": [
"type:wordpress-muplugin",
"cedaro/satispress"
]
}
}
}