a variable for the week number of the current year would be a handy addition

Request accepted. However it is not quite as trivial as it sounds:

http://www.blackwasp.co.uk/WeekOfYear.aspx

The first day of the first week is not always the first of January, so it is
likely that we will have to come up with more than one variable for this
representing the various trains of thoughts of when the first calendar week is
starting.

According to http://en.wikipedia.org/wiki/Week_number#Week_numbering , at least
six systems are in use (of which three are listed there), and the system used
in US and Canada differs from the ISO standard.

Created an attachment (id=7316) [details]
Test Version - Pre272

Here is a test version that implements week numbers based on the rules of ISO
8601. Usage: %cwIso(offset)% , where 'offset' represents what you count as
first day of the week, based on the following:
0 - Sunday
1 - Monday
2 - Tuesday
3 - Wednesday
4 - Thursday
5 - Friday
6 - Saturday

so typically, in Europe you would probably use %cwIso(1)%, in Northern America
%cwIso(0)%

Test Version - Pre272

this Version don't work for me.
On new messages i get our Prompt window "Cant't be used for new Messages" and
on reply the fields stay empty - german Windows 7

Created an attachment (id=7317) [details]
0.9 pre 277

this one should work better in any locale. there was a problem in the
localization function with potential to impact which is now much improved.
localization is now based on Operating System Locale and doesn't try to guess
Application locale.