http://youtu.be/mw_put5cfs0
Is this a regularly used counter (high percentage) to the lockdown, gi or no gi? I tend to like the counters that involve u sitting on your hip and bring your trapped foot towards there hips. But maybe this is something I should try. By the way, I have noticed several white belts all of a sudden working on lockdown/electric chair set ups, so I wouldn't be doing this kind of a counter on them. 
I peel the bottom foot off with my free foot and then swing the previously trapped foot to my ass. It very rarely doesn't work.
Lockdown prevents you from advancing. Instead of trying to go forward, go backward (south) and the lockdown loses it's effectiveness. In many cases after you go south just a bit you can reverse and advance directly to mount.