Comment by structural

Have you considered using GStreamer pipelines for this? And taking it one step further, if you want to use OBS generally, there's a plugin for OBS that allows you to integrate pipelines too.

The common case for the plugin is to do some preprocessing before importing the video into OBS, but you can also use the pipeline as a video filter, which is really powerful, because you can inject a filter into not only a single source, but also at any level of a nested scene. So you can scale/composite several inputs together and then the video filter can copy the source stream and encode/tee it to a file output. Might still be considered a hack of sorts.