That's a neat way way to tight together both worlds, and I can see it being useful in cases like testing.
Nonetheless, it is important to distinguish the need to communicate between programs and the need to programmatically run a piece of software like ffmpeg and getting its output.
For the seconds case, especially in more complex architectures, where you need "interact with software written in another language" it makes sense to explicitly separate this interaction, for example through a broker [0]. In the end, all you need is a way to communicate from Program A that Program B can do some sort of job, and this can be a simple string pointing to a raw video file in a storage like S3, not necessarily the raw file.
Nonetheless, it is important to distinguish the need to communicate between programs and the need to programmatically run a piece of software like ffmpeg and getting its output.
For the seconds case, especially in more complex architectures, where you need "interact with software written in another language" it makes sense to explicitly separate this interaction, for example through a broker [0]. In the end, all you need is a way to communicate from Program A that Program B can do some sort of job, and this can be a simple string pointing to a raw video file in a storage like S3, not necessarily the raw file.
[0] http://www.artur-rodrigues.com/tech/2015/06/04/beanstalkd-a-...