You need to read up on making an initramd that will contain the necessary bytecode. Module loading happens later on in the boot process so your framebuffer would kick in pretty late. I'm not sure what you have to do on slackware, read up their documentation and see what scripts you're given. The only thing I have on the matter is for Gentoo

http://forums.gentoo.org/viewtopic.php?t=49036

and there's another thread for using gensplash around there somewhere. I'm not sure if Slack has a channel on freenode [irc.freenode.net].

Cheers!