I actually went ahead and installed Slackware x64 on my second hard drive /dev/sdb in my MacBook (swapped out the DVD-RW drive a long while back).

After following the process to create a 100MB EFI vfat partition, ~460GB Linux filesystem and ~4GB swap file partition, then installing Slackwerwisare x64 and creating a bootable USB stick, I eventually got the system to boot into Linux without even using the boot stick by copying specific files from the root partition on the USB stick and the /EFI/BOOT folder to respective locations on /dev/sdb.

As to the original issue, I did get to booting Slackware in the VirtualBox VM, but was having weird issues with XOrg starting up. Nothing wrong with .xinitrc or .Xsession or permissions, etc.

Either way, now I have Slackware running (standalone), and still need to tweak network config, but I am otherwise good for now!