Just don't include the drivers in the kernel, and don't list them in /boot/loader.conf. That way, the modules won't be loaded automatically at boot time.
If this is a single-user system, then you can write a script that will kldload each of the modules, and put that into the Autostart directory for your window manager/DE (KDE uses ~/.kde/Autostart/). Any executable scripts in the Autostart directory will be run automatically as part of the login process.
If this is a multi-user system, you can write an RC script that will call your script as part of the "start" section. Be sure to put kdm into the REQUIRE line, and possibly into the AFTER line, so that it's not run until after kdm is running.