I think that needing to build a screen and keypad to enter Wi-Fi details is not how I ever envisioned an internet of things device working.
Something along these lines seems more logical :
Provide a webpage on the Photon where you can manage Wi-Fi connections. Connect to it directly through any Wi-Fi enabled device and add the local network. View any live information on the Photon webpages directly - negating the need to build a screen on the device. Add any configuration screens on the Photon to manage settings.