There is lot of overlap in how the film industry and the tech industries work - and startups and indie films are a great analogy!
The use of Angel as an investor term originates with Broadway shows, and the “Broadway Angels” that provided investment capital so “the show could go on.” Anyone who has been in the entertainment industry knows that most shows lose money. But, that didn’t bother Broadway Angels – they invested to promote the art form, rather than merely to make money.
Sources:
http://foundercorps.org/node/67
http://www.gsb.stanford.edu/ces/resources/angel_financing.ht...
If you don't like the analogy, pick another one. But more broadly, there is a lot of similarities between the types of people involved. I've advised creative, driven indie filmmakers who fit right into the creative, driven mould of startup entrepreneurs.