Is there some de facto "standard" what password would work, if someone wants to keep open WLAN access point but still has encryption set ON?

They are changing the law in my country, to allow connect all un-protected WLAN-networks without needing to ask permission. It is assumed by the new law, that anyone is capable to set up encryption and password if wants to keep his/her WLAN private only. If the WLAN is open, it is assumed the owner lets others to use it.

But I think, there should also be standard password to try if someone wants to provide also public WLAN access point, but still wants random "customers" to use encrypted connections.

If not such standard exist, RFC-draft:
I'd suggest "guest" as a password, or if longer needed "guestguestguest" and if only numbers, then 48378, or if longer deciman or hexadecimal needed, then repeating until enough 4837848378483. "48378" is "guest" in T9-code.
(Right now I do not remember what all kind of passwords would be needed in all WPA,WPA2,WEP,*.)