It's possible to override a messages in a FTL template by specifying a custom i18nbase.
We could provide a different solution since the base name is deprecated.
It could be a key generator in the MessageWrapper which would generated the keys:
FTL: i18n[my.key]

public String get(String key) {
        return translationService.translate(localeProvider, base name,  
                               Components.getComponent(TemplateKeyGenerator.class).generate(key)});
}

class TemplateKeyGenerator {
     String[] generate(){
         return new String{key, site name + key};
     }
}

It would be possible to define a custom TemplateKeyGenerator via module descriptor. E.g. Site module would bring a SiteAwareTemplateKeyGenerator.
Be aware that this is different than other info.magnolia.i18nsystem.I18nKeyGenerator s since those generates keys from nothing. Here we need to generated additional keys to the provided one.