Mautelino,
I remember one lucid dream which incorporated mindfulness of breathing, a few weeks back.
Here's my experience: I had my eyes closed in the dream. I felt something rushing at my face repeatedly. Aware that I was dreaming, but unable to wake up, I decided to focus on the breath. During the day, I try to become aware of the breath as often as possible, and somehow, this carried over into the dream. I was able to find the breath, and was about to do my usual dedication of merit for the meditation, when I thought "I'll dedicate the merit to whatever keeps rushing at my face". I was able to concentrate on a few breaths, then woke up.
I think my habit of "finding" the breath as often as possible during the day somehow helped me find it in that dream.
Since we dream only part of the time during sleep, and lucid dreams are so rare (in my experience at least), I doubt that regular practice and an accumulation of "cushion hours" is possible using lucid dreams. But the experience is certainly very unusual.
Dreams are really interesting. "We're all schizoid in our dreams" (semi-remembered quote without attribution) The other personalities we interact with while dreaming are, after all, produced by the same psyche as the one we identify with. Anatta for materialists

Cheers,
Florian