Setting diagram style through Applescript ?
 

Can't see this in the Applescript dictionary for Omnigraffle - is it accessible ?

(Would like to bypass the Choose Diagram Style dialogue, which is works fine but is a little cumbersome).

A bump to a really really old post, is this possible?

While Diagram Style doesn't appear to be an accessible property of Document, Template is. It's cumbersome to set up, but you could make a template for each diagram style you'd like to use. (Template Chooser, New Template, Choose Diagram Style, Save.) After that you can say, for example,

tell application "OmniGraffle Professional 5"
set the template of the document of the first window to "Clouds"
end tell

(Note that you apparently need to relaunch OG before it can see new templates.)