Yes, i think it's fine to include that. We often get portfolios with images of things like dresses made from vintage patterns, or things like wedding/prom dresses which demonstrate experience with fabrics like satin or velvet or whatever. The portfolio doesn't have to be ALL stage work, since other skills are relevant.

And, the stage work isn't always "I made this whole dress." Folks also do things like include before-and-after fabric swatches they dyed, or paperwork they generated for show tracking as a wardrobe head, etc.

Glad to know that the blog is a good resource and again, best of luck!