What exactly is your use-case? To save the _file_ so all the on_save changes (tabs, trailing spaces, newlines, plugin changes) and conversions (BOM, encoding) get done or to save the _buffer_ which is just writing the bytes somewhere to be reloaded later, somewhat like a bulk undo.
The former needs to be done in Geany, and because the whole file writing thing is messy and convoluted would probably (from only limited evaluation) be a fair bit of work.
The latter (and the reload) could easily be done in a plugin, if there is enough demand maybe even the core `saveactions` plugin.