Using header-args to set default TODO state?

[Michael Maurer]

I've been wondering if it's possible to define a default todo-state
for all items appearing under a headline, like

* shopping list
:PROPERTIES:
:header-args: todo-state: TOBUY
:END:
** bananas
** bread

so when calling org-agenda both bananas and bread have the todo-state
TOBUY applied to them. Obviously placing "todo-state" after
:header-args: doesn't work, it's just an example of how I imagine it
could work.