Investigating the connections between mindfulness meditation and the brain's serotonergic systems seems like a promising avenue of research. Mindfulness may be a way to directly influence this system based on observations from several domains.
1) A bedrock principle of psychopharmacology is that increasing the amount of serotonin in synapses improves depression and anxiety.
2) Acute doses of HT2A agonists (e.g. psilocybin) can also improve depression. These agents produce brief and intense sensory experiences. At low doses, subjects do not report hallucinations, but do report that sensations seem more intense and more affectively pleasant (e.g., colors are brighter).
3) Mindfulness, which is focused concentration on sensory input, has been shown to be effective in reducing depressive symptoms in RCTs (van Aalderen et al 2011).
4) Therefore, concentrating on sensory input as with mindfulness may produce similar effects to the SSRIs and HT2A agonists, mediated by the same pathway.
In a sense, giving SSRIs (or the more powerful one-time punch of psilocybin) may produce an exogenously-created mindfulness. No research has yet been done on the involvement of serotonergic circuitry with mindfulness meditation's effects.
Many of the measures correlating mindfulness meditation to outcome concern decreased rumination. To that end, the introspective among us should take note of this quote from a 2013 paper by Paul et al in Social Cognitive and Affective Neuroscience: "Our results suggest non-reactivity to inner experience is the key facet of mindfulness that protects individuals from psychological risk for depression."