Hi Stephen,
I have some VBA code which I use to delete the existing border and title block from an existing drawing and replace them (as well as sketch symbols) from a defined template (as well as updating the documents styles to match my local styles)
The code was initially created by someone at my parent company so some of it I don't quite understand but I managed to manipulate it to do what I needed it for. Hope this may be as some use to you!
When it gets to the part where the file location of the template is specified, you could add a pop-up box where you then have a selection between the different templates you want to be able to choose from.