Pseudo Guide to launching Roblox externally.

RobloxPlayerBeta.exe gets placed in a new directory every time roblox gets updated.
To get the name of the version folder, just pull this from the website:

http://setup.roblox.com/version

There are two places it can be located:
%LocalAppdata%\Roblox\Versions\CURRENTVERSION\RobloxPlayerBeta.exe
%ProgramFiles%\Roblox\Versions\CURRENTVERSION\RobloxPlayerBeta.exe

If you can't locate it, pull this exe from the website, dump it into a file and run it, then check both locations again.
http://setup.roblox.com/Roblox.exe

Once you have exe, you just have to start a process with a --id parameter (where --id is the placeId you want it to launch)
For example this would launch Crossroads:
RobloxPlayerBeta.exe --id 1818