How could there be a placebo for meditation? A placebo has to replicate the experience of going through the 'real' treatment being trialled. How could you create the experience of meditation without the subject actually meditating? The claim being tested is simply that the experience of meditation is beneficial, so I don't know what form a placebo could take.

It would be OK to do a trial where one group is instructed to meditate and the other is not given any instructions, and you check back with each group in a couple of weeks and ask them how they feel. If there's a difference in the meditation group, that proves there is value in meditating. Placebo doesn't come into it, because no one is claiming there is any benefit to meditation that doesn't come from simply experiencing it.

For comparison, I once heard of a way of testing acupuncture against a placebo – someone invented needles that look and feel the same but don't actually penetrate the skin. So they could compare the reported benefits of 'fake acupuncture' with real acupuncture, and see if there's any health benefit to the physical intervention, or if it's all just theatre. And if it is all just theatre, that's OK – it doesn't negate the health benefits of acupuncture, it just helps us understand that it's all about the experience of having someone slowly and dramatically stick pins in you, which is interesting. It's not the same as saying that acupuncture doesn't work.