Thanks for asking our advice. Hopefully we can steer you in the right direction.
Like you, I have a fetish for long hair. I have also been married to my wife for almost 32 years. During our time together her hair has been shoulder length, pixie cut length, and now about bra strap length.
When we first started dating I didn't come out and tell her that I have a long hair fetish. I remember once when she was giving me a blowjob that I told her that her hair would feel good. She looked at me kind of funny and didn't seem to understand.
A few days later she told me she had a surprise for me and told me to close my eyes. When she came back in the room she had the trimmings from the ends of her hair that she had done recently and proceeded to try to massage my cock with those little ends of her hair, which didn't feel particularly good. So there was a learning process.
Over the years she has come to appreciate my hair fetish. We usually start off our lovemaking with her brushing her hair over my body and my groin area.
My best advice to you would be to make it about her and her hair, not about a fetish for long hair.
I would avoid the use of labels like hair fetish or long hair fetish. Simply compliment her on her hair, tell her how beautiful and soft is. Tell her you love how it feels against your chest when you are cuddling after sex. How it makes your skin tingle. Give her time to figure out for herself that it would feel great against your cock before you ask for a hairjob. If you do have to ask for a hairjob, promise not to come in her hair. There's time for that later.
Also, be willing to help with her hair care, whether it's brushing or washing or whatever.
When you do these kinds of things, she may want to grow her hair longer. But if she doesn't you can still have a lot of fun with what she has.
Best of luck! Let us know how it goes.