On the one hand, I think it would work. But to be honest, I can't see a 2 year old being able to put it on by themselves, which makes me think it would get limited use. My dd's interest in dress up has really coincided with her ability to put clothes on herself.
I understand your concern about stuff around the neck. But I think you would be better off using a piece of velcro to attach it around the neck - obviously leaving a generous neck opening. Or even using the snaps. Also - be careful about length. Make sure it ends around waist length - you don't want the kiddo to step on it and trip.
If you go in the sewing thread, go back a couple pages and we had some discussions on making capes. There were some instructions posted, as well as some links to instructions and patterns. IIRC, the discussion was right around Halloween time.