After some searching, I was able to get network bridge and network bond working.
During the research period, I noticed that some distros supported bridging and/or bonding through their network init scripts, so my question is this:
Does slackware support network bridges and/or bonds through rc.inet1 / rc.inet1.conf?

I have bridges setup via /etc/rc.d/rc.local which calls a custom /etc/rc.d/rc.bonding script I created. I bring up these interfaces in VM's so I leave them out of /etc/rc.d/rc.inet1.conf on the host. It would be easy to add the bridged interface to the Slackware system if so desired.

I have found this about setting up bonding on Slackware - it does involve /etc/rc.d/rc.inet1.conf in it's configuration. This uses the Slackware networking subsystem to bring up the bonded - (or is it "bound" perhaps?) - interface.

In general the /etc/rc.d/rc.inet1 script will bring up network interfaces you add to the etc/rc.d/rc.inet1.conf so for bonded and bridged interfaces those interfaces have to exist before the script executes.

Robby Workman hosts a rather clever script to bring up a bridged interface here which is called from rc.modules.