Found a workaround here: https://groups.google.com/forum/#!topic/pandoc-discuss/3DP8NACzWjY
Everything works as desired. Maybe it would be good to add it to manual?

воскресенье, 18 июня 2017 г., 13:42:01 UTC+3 пользователь Uberslowpoke написал:

I have pretty same question. I use proof environment, and I want to interpret its content as markdown.

суббота, 1 февраля 2014 г., 23:54:35 UTC+4 пользователь hff...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org написал:
Is there a way to have pandoc interpret the material between the begin and end tags of a latex environment as markdown, not latex?

As an alternative: Is it possible to run, say, a perl filter, calling pandoc as an external command for transforming the replacement pattern, e.g. from markdown to latex, in the process?