First off I applaud your initiative and getting things done.
Now I'm going to play devil's advocate.
Did you look into the reasons why the room was closed to open booking?  Were people taking advantage and camping out there or planned meetings not happening because of double booking?  We have an executive board room that is booked by the CEO's admin only, however she is very responsive if we need it.  We also have many many options for meeting space in our building, but there are times when a certain size is needed and just isn't available.

How did she refuse to discuss directly?  I would have cc'ed her boss and my boss on an email and meeting request to discuss the situation before going around and getting access.

Remember in the end we are all supposed to be working for the common good of our companies.

Now having said all that - yes I have worked wth control freaks and "islands" in companies.  Find the best work around that all keeps the peace.  That person is most likely not going anywhere.

I have a couple of thoughts
First your boss probably had a million other things on his mind and didn't realize how he sounded. Try not to take it personally.
Second, this doesn't mean don't  take initiative, but I would get more input from your boss for direction if possible without looking needy.
Third, if this still bothers you at the end of the week plan a time to discuss this and other items with your boss, not singling out this issue.
Good luck