How can I see what custom URI schemes are registered?
I like to use RCDefaultApp to manage handlers for protocols and URIs.  There may be a better way to do it programmatically, but Rubicode's RCDefaultApp is very useful for a lot of other reasons too.

When you dump the launch services database does it show up there?  

 /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -h