From my experience there are two ways to do a foam lay-up. For a one-off you can lay the foam on the inside of the forms or on the outside of the forms.
On the inside of the form method it allows you to do your foam lay-up and first glass them remove the hull half from the form and easy access to the outside of the hull. It takes more care in the alignment of the forms on the strong back as it is harder to visualize the same and form of the hull when looking inside of the forms. When planked out with foam you can see the inside of the hull but again you cannot see the outside of the hull and look down the hull for fairness. This method I believe is easier for the one off boat. It easy to attach the foam and for the most part it stays in place although it wants to curl during curing, I found out the hard way.
Foam on the outside of the forms is much more difficult to attach to the form as you working under and inside the form on a narrow hull. Also you must deduct the finished hull dimension from the form size or you will be adding to size of the hull. If this is done during the lofting it will be easier but often the plans are drawn to the finished size - Just something to think about in the early planning stage.
Now if I were just building a plug I think I would just attach the foam to outside of the forms as it much easier to see the fairness as you go along. You can run the screws into the foam and fare over them. The only thing is that you are putting all your eggs in one basket in hopes that not going through a prototype boat your design will be a good one. You are putting time and money in the plug and mold for a untested boat.
Either way good luck with your project it is a lot of work and fun if you like that sort of thing. I yeah I almost forgot to mention it takes up most of your free time also. Bad lat if you would like some more information about what I have learned to do and not do feel free to contact me. Iíve learned much during my endeavourer.
The Man Shed