I have a simple suggestion, may not be that simple to script, but it could be worth it.
Store acces to a private forum in a database instead as Cookie on the computer. Its frustrating to fill in forum passwords ever 3 days or so, and not only on PC, but also laptop, on tablet and smartphone. I only have acces to a couple private forums and even I find it frustrating.
Some people here even need a whole list of passwords because of their thousands groups they are part of, wouldn't it be a lot easier to just have it stored permanently in the database? If the forum password changes, acces should be removed.
PS: You can't let google chrome keep the password, because it will only work for 1 private forum.