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.
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.
Thank you for letting me know. I shall certainly look at updating relatively soon!
Thanks for the suggestion. I've added this to my list of potential future features.
Potentially this is something that could be added by someone skilled in Scheme/JavaScript as you can also use Scheme macros, for example %(+ 3 2)%. Unfortunately the Scheme date-week-number function isn't implemented in the Scheme used by beorg which would have made this really easy...
Jacob Ilsø