I have far 2.0 installed to "c:\program files\far2" via the Far MSI installer. When I install "WinSCP as FAR Plugin 1.6.2" It defaults to installing to c:\Program Files\Far\Plugins. If the installer can be made to read from the registry, it could look for a Far2 installation via: HKEY_LOCAL_MACHINE\SOFTWARE\Far2\InstallDir, or, HKEY_LOCAL_MACHINE\SOFTWARE\Far2\InstallDir_x64 when the 64 bit version is ready.
I can examine the behavior of far 1.7 as well, but I think registry checks make sense for guessing the install directory.
Also, on my machine the installer should have defaulted to "C:\Program Files (x86)\Far\Plugin" if the installer was getting the program files prefix correctly.
Thank you for your work on WinSCP, the far plugin is a great asset to my work.