btw I just went to the overture search suggestions tool
1081 searches for long hair fetish
385 searches for short hair fetish
So, to answer your intial question, yes there is a market for the long hair niche. However, there is still a smaller more targeted market for short hair
btw here is the google searches for both so you can see what is out there:
http://www.google.com/search?hl=en&q=long+hair+fetish
http://www.google.com/search?hl=en&q=short+hair+fetish
I guess ultimatly the question is - why not do both and cover your bases? If I didn't already have a back log of 'must do' dream projects I would do the short haired one myself ;)