Well, I don't think festivals are going to try and control your aspect ratio, color or film look. Art is subjective, and they get that to some degree, enough to accept a non standard look.
Something that I would advice is considering your options when it comes to producing that analog look. What a lot of people, including studios do to split the difference these days is to just film on a strong modern digital camera, and then create the analog look in post.
The reason is that people have really become attuned to HD and 4k at this point, and you can still get the feel across with the post effects, without having to sacrifice so much quality.
Do you need it to be super low fi for the concept to work? Like this?