"Backstage" generally covers jobs like Runner and Runway Room. It depends on whether you approach a production company working with a fashion house or the fashion house itself.
A sponsorship volunteer is generally someone representing and 'greeting' for corporate sponsors investing funds in a show and a collection. You'll stand around either showing people into their seats or pouring their drinks for them backstage. Waste of time, IMO. You won't really get to see the nuts'n'bolts of fashion shows, which is, presumably, the aspect that attracts you.
Many fashion houses will certainly take on extra help before and during showtime on an internship basis. Bluntly, this means 'no pay'. However, it's a very good introduction to how things work. You can learn quite a lot doing this. I don't know what your eventual aims are but would suggest that you get in touch with designers and houses to ask about short showtime internships. If knocked back initially, persevere. Someone out there will be glad of some keen - free! - help.
It looks very good on resumés and CVs. Go for it! Be prepared to go to NYC or even further afield, though.