[O] agenda item formatting

[Eric Abrahamsen]

In the past I've been pretty happy with the default Agenda formats for
display todo items and such. It used to bug me that you couldn't see the
lineage of the items in the todo list, but then I discovered
org-agenda-prefix-format, and the %b escape for breadcrumbs, and that
was good enough.

Now I've got a couple of custom blocks where I *really* want to control
the formatting exactly, and I'm having difficulty doing that. The
docstring says you can use this in org-agenda-prefix-format:

%(expression) Eval EXPRESSION and replace the control string
                by the result

I don't know how I'm supposed to use this. I can put a custom function
in there, but the function is passed no arguments, and is `eval'ed in a
nil environment, so I can't access any dynamic variables. Apparently all
it can do is return a string that is unrelated to the item being
formatted, which isn't very useful! Probably I'm wrong about that, but
I've tried for a while, and can't get it to do anything else.

Is there any other way to access the final formatted string for an
Agenda item, and doctor it?

Thanks!
Eric