Ok, well that's just better than fantastic! I now have master template display conditional upon whether or not the user is logged-in, which is even better than I hoped for!
So if the site is viewed by 'the public' all they see is a full-screen website, but if a user with appropriate access is logged in, they see the full xowiki layout plus OpenACS header as usual.
To do this add:
<if @admin_link@ not nil>
<master>
</if>
<else>
<master src="/www/blank-master">
</else>
The same conditional can be applied to the @top_includelets@ and @footer@ variables if required.
I used the xowiki template parameter to set the default template to the new 'full-screen' template, as adding {{set-parameter template_file full-screen}} to the template was not effective. I also tried {{set-parameter full-screen 1}}. I imagine therefore that passing arbitrary parameters is not supported and would require custom code in the page rendering method.
One surprise I encountered was that when I removed the header info from my template and put [[my-css.css]] into the template section of the xowiki::Form, instead of adding the css file to the document header, a link to the css file was rendered on the unstyled page. Not sure what I've done wrong there but as a workaround I added the css file to the xowiki instance package parameters.
Regards
Richard