I found the old ShellExec.lss lotusscript library somewhere. What I did is get the attachment as NotesEmbeddedObject, extract it to the filesystem (C:\Temp for example), pass the file path/name using the agent.runWithDocumentContext() method to the ShellExec.lss to launch it.
At least it worked on my platform (Win7x64). I'm 100% sure it won't work on others though.