I think a module could make these changes for D6. If you add one more feature, I think it will be a hit. I want to default to whatever login mechanism the user last used on my site. So, storethat in a cookie and make the site react to the cookie. If the user last logged in via OpenID, show the OpenID form.
I wonder if some clever jquery that runs onload would be simpler than the procedure you describe here. Just simulate the click for ‘login with open id’.