the developers told me that there is no Bela-specific code in the Soul binary. The Soul binary is a JUCE project, built for several platforms, one of these is linux ARM, which includes the Bela support (when built with the flags mentioned in the other thread). Therefore, support for buttons can be added in the JUCE mainline repo, for instance turning them into audio signals or into events, and this will automatically be inherited by all JUCE projects, including Soul!