Siebe said: “Cultivating metta has effects on the heart chakra, for example. Seeing tilakkhana leads to a process of grounding, becoming less dreamy, an effect of sobering up.”
That is correct.
Siebe said: “Some buddhist schools have invented special purification practices based upon the understanding of these subtle processes and how they are influenced.”
That is correct too. They are deviating away from the key message and that could be distraction. It is not necessarily bad to learn about such things, but there is a danger that one could get focused on those chakras, nadis, etc and spend too much time on that.
Siebe said: “Would the Buddha disapprove? I do not know for sure ofcourse, but why would he if they are conducive to the goal of reaching Nibbana?”
If the Buddha thought those things are necessary, he would have taught them. Again, only a Buddha can teach about the samsaric suffering, whereas there are others who can teach such mundane things. He did not want to deviate from the key and critical message.
The Buddha knew much more than current scientists about nuclear power, lasers, etc. Those things could be useful to a society at a mundane level. But they are not necessary (and could become distractions) in pursuing Nibbana. If one understood the true dangers of the rebirth porcess, one would not spend much time on such topics.
However, when one is sick, one MUST seek medical advice. Without getting better, one cannot pursue the Path to Nibbana. So, we need to keep that in mind too. If one can get rid of some bodily aches by practicing yoga or just to maintain physical health, it is prudent to learn and practice yoga. I myself do yoga, but I can learn those techniques from websites/books specialized in yoga. There in no need for me teach yoga or to discuss those techniques here.
So, let us not mix up Buddha Dhamma with yoga or chakras or nadis.