Forgive me if I'm wrong, but a leaderboard usually means running a server, or using a service like Clay.io.
I would also guess that you could make one for yourself using the multiplayer object.
Running a server is quite a hard choice for the beginners, and Clay.io doesn't work well in CocoonJS. I had emailed Austin for a week with this issue, but nothing improved. It ended up with showing black box on the screen. Sometimes it works, but mostly not.
Hmm. Clay.io... Well, I have sticked to CocoonJS for the ads, so I didn't test it on the Crosswalk. Maybe it works well in there because it's based on Chrome. Same for multiplayer object too. I'm not sure, just guessing.