Questioner: Can I practice Zen and be a good Catholic?
Roshi: If you practice Zen you can, but if you practice Zen Buddhism you can’t. – Philip Kapleau
There is more to the answer, but the above is most instructive.
If we changed the question to, “Can I practice Zen and be a good Humanist?” I believe the answer would still be the same, “If you practice Zen you can, but if you practice Zen Buddhism you can’t.”
Humanism is a worldview “without supernaturalism.” Gods, devas, heavens, hells, karma, and rebirth are supernatural claims that Buddhism makes. They are claims that any secular Humanist would reject.
So no, a good secular Humanist can’t practice Zen Buddhism. But a good secular Humanist can and should practice Zen. There is evidence from science on the benefits of meditation.
So what do you call a Humanist who meditates? A Zen Humanist.