There isn't one that I'm aware of. This being LFS, you would choose which window manager or desktop environment you wanted to start by manually changing the settings in $HOME/.xinitrc - as mbostwick said - according to the relevant configuration instructions in BLFS. That's assuming, of course, that you want to keep with Slackware's tradition of defaulting to runlevel 3; if you later decide to default to a graphical login, display managers like GDM and KDM can allow you to choose your desktop or window manager from a session selection menu on startup.
You could also use xwmconfig itself on your LFS system, provided you make sure it matches your filesystem layout and that you have the necessary files in place. It's just a shell script that checks for the existence of global xinitrc files in /etc/X11/xinit/ and then either creates a global symlink in the same directory or copies the appropriate file over to your home directory, depending on whether you run it as root or not. There's a slightly more detailed explanation
here (the 1st four paragraphs) and you can find the actual script itself
here.
