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.