If I understood Gary's approach properly...
I don't think it would require a interior redo. Just cut out a small rectangular spot of ensolite and glue in a piece of plywood to screw into in its place. If the plywood is painted to match (and especially if covered by the curtains!) it shouldn't be too noticable.
I used a similar approach in my PlayPac. I glued in (construction adhesive) plywood blocks (which I later covered with carpet) to attach curtain rods to. Worked fine. The blocks were less than 1"x2". Just have to make sure they're in the right place.