OK, All Ye Who Mock

So I mentioned my bathroom project where I temporarily rerouted the water to fix the toilet.  Almost immediately, my many admirers called me an idiot for not shutting off the water to the entire house since the inline to the toilet wouldn't completely close.

I'm stupid but not an idiot. I thought of that!

I went to in the front flower bed and found it. It's new school and simple. It pretty much is this:

The house is probably 12 years old or so, but I bet that shut off value has never been turned. Why do I believe that? I couldn't budge it! I used WD-40. I tapped it with a rubber mallet. I used tools for leverage. Nothing.  I would have used a vice grip but there is not as much room on the neck as the one depicted here.

But given the option of snapping that thing off and/or having Old Faithful come to life, I let well enough alone. I'd used PVC pipe and comedy to fix the toilet before I'm screwing up the water supply to the entire house.

So here's my question: Are there any tricks to getting that thing to turn? I really don't want to call a plumber for that but it needs to work.  One of these days I'll have a busted pipe and that won't be the time to try and figure out how to get that thing to turn.