It took me a bit to figure out how to get the Brother DS Mobile DS-620 to work on Slackware. But after some digging I figured it out, and it's pretty simple.

1. Go to the Driver Page and get the rpm file. If you want to skip all that clicking or if the page is down, here's a Direct Link (Archive.org), but be warned, it may not be the latest version if you're reading this in the future!
2. Become root, and run this command on the rpm package, this will create a Slackware package from the rpm:

rpm2tgz -s -n -r -d /path/to/download.rpm

3. Once it's done being converted to a Slackware package, run installpkg /path/to/output-package.tgz, this will install the package to your system.
4. This is a not so obvious step, but you need to add /sbin/modprobe sg to your /etc/rc.d/rc.local. This loads the generic scsi driver, which is neccesary for the scanner, for some reason?
5. Open /etc/sane.d/dll.conf in your preferred text editor, and add dsseries to the bottom. This will make SANE notice it.
6. Reboot your computer, or run this if you don't want to reboot:

udevadm control --reload
udevadm trigger

7. Add your user to the plugdev group, if you want access without root. I don't know why the device is owned by plugdev. In theory the udev rules should make it owned by the users group, but in practice, no.
8. Verify your scanner works with scanimage -L.
9. Once you got all that, you can open skanlite or skanpage and begin scanning pages.

This is quite a bit of steps, but thankfully is just a one time deal, and I wanted to write it down for future me and others who want to use it. Who knows, maybe we can convince Patrick to add this driver to Slackware itself... haha just kidding... unless?