I would take a look in the next few days.
Because I´m trying to implement some of the security options mentions in https://httpsecurityreport.com I will try to solve this topic http://forum.getkirby.com/t/more-secure-php-session-cookie/1828 too.
Update:
The code is ready to be pulled: kirby/pull/294
And there is an corresponding docs update as well, linked to the pull request.