You can scan, and choose which one to connect to by yourself
Get scan results with signal strength and mac using
iw dev wlan0 scan
Parse it in a script and select which one to join manually. iw scan results would contain "RSN:" or "WPA:" or "WEP:" if they are not open. So you can wee out encrypted networks, and select one which is open, maybe with best signal strength "signal: -45.00 dBm".
Since its openwrt you can do it in a Lua script maybe.
buf = output from iw scan
buf = '\n'..buf..'\nBSS' -- .. is concatenate in lua
Use this regex to match a single block (in Lua or anything else):
"[^A-Za-z%d]BSS" -- %d is all digits in lua, maybe 0-9 otherwise
Within each block grep SSID, signal, and check if "[^A-Za-z%d]+RSN:", "[^A-Za-z%d]+WPA:", "[^A-Za-z%d]+WEP:" have any match, ignore the blocks which do, and ignore the ones with the SSID in your blacklist.