Thanks for the suggestion. I've added this to my list of potential future features.
Thank you for letting me know. I shall certainly look at updating relatively soon!
Thanks for the tip about using Scheme; I bet I can make that work. However, I second the request for %week% to be explicitly supported, since org-mode's file+olp+datetree function supports ":tree-type week" as an option, and builds the tree using week numbers.
The latest version of beorg does have the latest version of BiwaScheme included. As you say though supporting %week% would be a much better way of doing this. I've added this to my feature list.