Open a Document on Disk.vi converts the document path to a string, and passes that string to vi.lib\Platform\browser.llb\Open URL in Default Browser core.vi. On Windows, this VI makes a callback into the LabVIEW.exe to call an internal function to open the document. I'm guessing that this callback is only supported on Windows, which is why Open a Document on Disk.vi doesn't exist on Linux. If you dig into Open URL in Default Browser core.vi on Linux, you'll see that it takes a completely different approach. If the 'htmlview' app is present on the system, it will use that. Otherwise, the first time the VI is ever run, it prompts the user to specify an application to use to open documents, then it stores this info in a preferences file for later use.
So for your case, if you're wanting to specify a custom application to open documents, I suggest either programmatically or manually modifying the [LabVIEW Data]/.lvbrowser file to point to the right application, then use Open URL in Default Browser core.vi to open the document.
