This is non-trivial, as it requires C# bindings for IOKit in order to send the necessary events. There does not seem to be an existing comprehensive library that does this; others have only bound the parts of IOKit that meet their application's needs.

Besides that, OS X users are highly likely to have media keys available on their system anyway. As such, this is a low priority issue.

References:

http://stackoverflow.com/questions/10273159/osx-programmatically-invoke-sound-level-graphic