AFAIK a device (a wifi-enabled RPi) with Zero-config networking enabled should be accessible via HOSTNAME.local without first having to join a network.
I have several Raspberry Pi running Jessie, with Edimax wifi adapters, and running Avahi. As expected, they are accessible from my Mac via HOSTNAME.local, HOSTNAME-1.local, etc... but ONLY if I add an entry to
/etc/wpa_supplicant/wpa_supplicant.conf so they join the network.
It looks like the issue is that there is no IP4 address assigned to the Wifi dongle unless it is configured to join a network.
My goal is to have mobile devices with a web configuration page that does not require first authenticating on a network... eg, a devie that is accessible anywhere with or without an existing wifi network by browsing HOSTNAME.local.
Besides Avahi (which is working), what else is required for a wifi-enabled RPi to host its own config web page w/o joining an existing network?