Wouldn't you instead use a Business Object Copy Rule on the Request BO in Window Manager to populate the Authoriser field as soon as the requester is picked. Then on the raise use field set the copy related attribute to true.
That would seem more appropriate that a template which as you've discovered can only take static values.
Thanks for your help here. I hadn't really played around with Copy Rules before, so good to know they are there to use. It didn't go exactly as planned, as I would have like to be able to use it on the "Initialise" event, but it appears you are limited to CurrentUser and CurrentGroup when using Initialise. I was able to tweak my process slightly to allow it to be workable populating the Authoriser user on a "Create" event.