Just sitting and focusing on breath (no chanting Buddha and Hindu deities and such involved). Is it allowed?
The short answer is no. Clearing one's mind can in fact be helpful in salat (prayer) and thikr Allah (remembrance of Allah, calling His name as worship). Not everything under the sun is micromanaged in the Sunnah, even less so in the Quran. Is it haram or halal to walk down the street? Take a walk in the park? Read books not written by Muslims? Go to art museums where nude women are painted? Give a baby a pacifier? Watch TV?
The Quran enjoins us to use our minds, to think. Afa laa t'aqiloon? Or "will you not think/ understand/ use your minds?" Is a frequently mentioned aya in the Quran. So we were given the ability to think and to comprehend for a reason and we are responsible to use them. We should not have a faith based on read-only memory, but one based on our own powers of reason and experience and that settles in "the heart" which in the Quran is the core of our selves and also our thoughts.
I myself have used mindfulness techniques and later found them immensely helpful in concentrating in salat. So it can be good. I suppose if you replaced salat with meditation that would be bad. If someone took a walk in the park to kill an innocent person that would be bad. It's what you make of it in that case. But in and of itself...I think you can understand.