Running php-composer as part of a build process

I'm porting a PHP application that uses composer[1] to download and 
manage what effectively becomes locally-bundled contrib.

Doing this during the build process isn't an option because it creates a 
nightmare of licensing and version management. Also, composer needs 
internet access and can be configured to do other things, like code 
compliance and unit tests.

Can I run something like composer from the post-install target?  Is it 
reasonable to assume internet access is available during port/pkg install?

1: https://getcomposer.org/