OK, since I got the thumbs up for publishing this code here goes.
I've missed having my own definitions in Skritter since I started using it this summer. Thanks to Google Chrome's easy interface I've now programmed an addon in javascript that injects your own definitions into the practice page. The data is based on a "database" in the js-file so nothing else besides Chrome and a little bit of setup is needed.
To get started you first need to enable extentions as per these instructions: http://googlesystem.blogspot.com/2009/03/first-google-chrome-extensions.html
Then download the zip-file http://rapidshare.com/files/319053361/skrittdef.zip and extract it to the "myextension" directory.
Finally update "skrittdef.js" by adding/removing entries in the last part where the "database" is located.
I hope there are not too many bugs since I've only used it for one day. So far it's working great for me, and has made studying Heisig with Skritter work like a charm.