As I understand it, if your KWallet password (assuming only one wallet) is identical to your login, it should be unlocked on login. Though I may be thinking of GNOME with it's PAM things here.
Unfortunately for me, it isn't doing so. I have three apps that seem to pass around the privilege of pestering me for the password on login - the Network Management plasmoid, Choqok and Kopete. Once one of them has asked, the others don't bother asking, and obviously once that one has the password for the wallet, they all get access.
It's especially annoying when the app asking is not the plasmoid, since fairly often it'll fail to connect, I have to restart the networkmanager daemon and re-supply it with the network's own password by hand before it'll let me connect.

So in a word - short of disabling KWallet, is there any viable way to get it to open by itself?

Hi, AFAIK it's not possible with kwallet at the moment. You're right that gnome-keyring does support this through a pam module.

the wish is here : https://bugs.kde.org/show_bug.cgi?id=92845 . it seems it might start working with 4.8.
For now the only workaround I know of is to set the kwallet password to empty.

Serafean