Did you work this out?
I am looking for the same thing,
"You have delivered [CALC FILED] this week"
I think the answer is "yes, it's possible." But, as you've noted, there are certain limitations with annotations and other options such as reference lines might have drawbacks. What this means is that while it is likely possible, there is probably not a single "this is the way to solve the problem" but rather a "this is how you could solve it in your case." So, to help understand our specific case, would it be possible to provide a packaged workbook (.twbx file)? You can mock-up data if necessary or mock-up a similar scenario using the Superstore Sales sample data provided with Tableau Desktop.
1) What Joshua said. Without having a better sense of your data, we can't give you an exact answer.
2) I put together some options for putting totals in different locations on the viz, in most of the examples in http://drawingwithnumbers.artisart.org/annotating-a-view-with-a-total/ you could replace the total calc with your particular calculation. However, not knowing what your data is like, your mileage may vary.
My Calc-F is this:
THEN 1 ELSE 0
Assets shipped are just dates (DD/MM/YYYY) so all this does count 1 for every record with in the last 7 days, then its a case of SUM(CALC-F). so i tried writing that in like the "Sum(number of records)" which is already in the annotation, but it returns ALL as a value where as it should be a number (89 as its happens so far this week!)
any help would be amazing!