Sorry for the delay in replying to this. I ended up getting the packages I needed from here:
OpenWRT package repository for mipsel_24kc
Those worked just fine with the 2.271 firmware as well. I had some trouble initially with getting them installed with opkg, and nearly just installed vanilla OpenWRT on the unit, but then I realized what I had done wrong.
By the way, for anyone else trying to do what I was doing (getting Avahi to expose “GL-MT300N-V2.local” on the WAN port), once you get the Avahi packages installed (I installed avahi-nodbus-daemon_0.6.32-3_mipsel_24kc.ipk and its dependencies), simply open port 5353 on the firewall, and away you go. Works perfectly!