Not sure about balena, most Linux based embedded projects I worked on were based on Yocto and all the source is available either through OpenEmbedded or some ve

Not sure about balena, most Linux based embedded projects I worked on were based on Yocto and all the source is available either through OpenEmbedded or some vendor consortium (e.g. Linaro). You only need to provide sources for the copyleft components you change and direct requests to the above entities for the rest.