[flak48]

Yep. I didn't do it with a RasPi but with a Chinese chip I found off AliExpress

Managed to strip down OpenWRT so it would fit in the meager 16MB RAM/8MB flash. In the init script (a bash script) just checked if a wifi network was configured previously (creds stored in a file).

If configured, try to connect to the network for 10 seconds. If it couldn't connect, then switch to AP mode and start a lightHTTPd server listening on 192.168.0.1:80.

The only files it served was a simple index.html with some JS that called a bash script via CGI that used a openwrt bash library to scan and list the available wifi networks.

Then a HTML form to choose an SSID and enter the password, upon submit call another bash file to connect to the network (switch to client mode) with a timeout. If it fails, the bash would switch back to AP mode.

Polling in JS to check if the connection was successful (by checking if AP mode becomes active again)