Brahmins Fought for Imam Hussain in the Battle of Karbala
It was not just Hindus, but also Christians who supported Imam Husain. John is one of the shuhada of Karbala. You only had to have the tiniest bit of morality and love of fairness, truth and justice to find yourself on the right side. The opposition at Karbala was perhaps the manifestation of the greatest evil mankind has brought forth.
There are Husaini Brahmins and Husaini Hindus too. There are many Ta'aziya and Alam processions taken out in India even today by Hindus. In Lucknow these processions are especially famous. In UP, at the Mazaar of a descendant of Imam Husain, Syed Salaar Masood Ghazi, to this day Hindus and followers of other religions come to pay homage.
Interestingly, there are also Qawwalis written by Hindus in honour of the Ahle-bait, especially Hazrat Ali. Since I am a big Qawwali fan, I have heard many of these... one which I happen to remember off the top of my head goes like this:
Ali ji, veer ho, balwaan ho, dharti pita tum ho
Jagat kay, eeshwar kay soorma tum ho
Anokhi tumhree leela hay koi kya jaanay kya tum ho
Kahien dharma tuma tum hu, kahien bharma tuma tum ho
Maula Ali maula...
Just goes to prove that Truth is respected by everyone, irrespective of religion, colour, caste or creed.