You can download a beta here : https://dl.dropbox.com/u/13064071/plantuml.jar
That allows this:
@startuml
while (check filesize ?) is (not empty)
:read file;
endwhile (empty)
:foo3;
@enduml
The drawing is not 100% perfect (work is in progress), and we are not fully satisfied by the syntax (which is slightly different from your suggestion).
Thanks for your ideas, and feedback still welcome!