Enabling autologout only for particular roles

We want to only turn on autologout for a couple of roles, and not have any timeout at all for everyone else. But there is no role that is equivalent to "everyone except the people who have one of these two roles". Disabling timeouts with a timeout of zero for some common role, like authenticated user, disables it for everyone, since people get the minimum applicable timeout.

The global timeout field help text says "Will not be used if role timeout is activated." I believe that is not true? In the help text for the Role Timeout checkbox, it says "Any role not ticked will use the default timeout value." That seems to contradict the previous line. Or else I am misunderstanding it.

Is there any way to do this other than patching the module code to default to zero rather than the global value for unchecked roles?