So I MAC Telnet to a new device, created bridge1 (vlan filtering not enabled at this stage), added bridge vlans, entered other basic setup.

On the very last step of the setup process, I entered:
/interface bridge
add name=bridge1 vlan-filtering=yes
and received the error "failure: already have interface with such name"

How do I enable vlan-filtering for an existing bridge on the CLI?

"add" will add a new bridge, which isn't what you want since your bridge already exists.

to change the configuration on an existing item, use "set":
replace 0 with the appropriate item number, which you can see in "print".