To me, scandalizing a gaff rig sail meant dropping/dipping the gaff, not hoisting the boom. I don't think anyone would try to hoist the boom when at sea. Trying to control the swing of the gaff is hard enough; to try to control the swing of the boom, as the ship rolled, would strain the gear. If the boom got loose, it could scythe the shrouds or backstays, leading to dismasting.
I suspect the model owner was trying to make the mainsail look the same as the boomless foresail. The brailing up of the boomless foresail, as seen on the model, was quite common. You only have to watch out for the flying blocks on the clew (which could be controlled by keeping the sheets taut) as the sail was brailed in; same problem exists when controlling a jib's blocks when tacking. It's possible to brail up the boomed mainsail the same way as the boomless foresail, if the main is "loose-footed" (ie. only attached to the boom at the tack and clew). The clewline is slacked as the sail is hauled up and forward. Brailing-up was a common way to reduce spanker area on squaresail ships; spankers were commonly loose-footed just for this reason.