With the stock behavior, no. That was a safety feature implemented to prevent people from farting around and trying to get private forums.
Reading private forums isn't hard to hack in. You would need to know the forum key, as you cannot count on always being able to read vars_forums., then it is a matter of assembling the path, and parsing the appropriate files.
Keep in mind, if anyone knew the URL to the page, your private forum topics would be compromised, hence the reason for the current behavior.
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