So... Yes. I have exactly this happen pretty often.

My wife has her hair currently down to her lower back. It's not as thick as I personally would have thought I'd settle for, but her genetics are not her fault and I adore her regardless.

She has known about my hair fetish since shortly after I met her 10 years ago. And allows me to indulge often. Normally, she'll keep her hair down for that very purpose. So I have easy access whenever I want.

But somedays... she'll plait it in a braid or do it up in a bun. On those days, I'll come up behind her while she's busy doing something she can't immediately stop. With her occupied, my hands start to undo her hair, even as she protests.

I've found that it's pretty easy to quiet her down from complaining if I give her hair a tug, bury my nose in it and just... Inhale deeply. She loves it. She asks for me to "do the nose thing" sometimes. It gives her goosebumps.

When I'm done undoing whatever she did to her hair, she'll silently make an annoyed face, but say nothing. Afterwards, as you said, she'll come to me with a brush so I can brush her hair for her, unless I decide to use it for a hairjob first.

The only things she asks in return for letting me indulge in my hair fetish and continuing to grow it out are that I wash it for her every other time or so and that I brush it when she's too tired to. Which are STILL things that I consider solid wins for me.