this.mappings["mymapping"] should be this.mappings["/mymapping"]
The "/" is required.
This old "PRERELEASE - 02/17/2012 GetApplicationMetadata" doc needs to be deleted ASAP!
This function was renamed to ApplicationGetMetadata in CF10 final build, and the new/correct documentation is here: http://help.adobe.com/en_US/ColdFusion/10.0/CFMLRef/WSf23b27ebc7b554b6c43bda313671e094bb-8000.html
GetApplicationMetadata() still works in code, but this is CF10 bug #3207841 filed at: https://bugbase.adobe.com/index.cfm?event=bug&id=3207841
In case anyone finds this old post, note that the function name did indeed remain getapplicationmetadata: