Steam beta shortcuts

Steam has built-in support for letting you specify command-line args for games.

Right-click the game's entry in the Library and click Properties. On the window that pops up, there's a button named "Set Launch Options...", and clicking that gives you a textbox that lets you type in command-line arguments that Steam will use when launching your game.

You can include the entries you would've had in commandline.txt in there (all on one line, of course, since the textbox in Steam is one line only).

No - this is not possible. Unless you launch an application or game from your Steam library, your Steam status will not update to reflect that the application (in this instance, your movie player) is running.

It is also not possible to create a custom status message on Steam. Adding your own games (or in your case, your media player) is pretty much as far as the customisation of "what you're doing" goes. This is something that has been requested as a feature more than once in the past, for example here on the Steam forums.