Because of the experience of others here who have already sent to Warsaw and received back by courier, I did a tiny amount of websearching and found this:
There are other reasons listed here, but note the date of 1 August 2012. It seems something changed by law and there should be no problem sending by courier company. Or at least I assume the courier companies would not be advertising openly this service if it was against the law
. I don't know if this applies to the Russian Post/regular mail.
So I guess the Embassy in Moscow is misinformed about this. I'll see what they have to say about it.
But at any rate, anyone in position / wanting to send should be able to do so in (relative) confidence. And anyone being told by Warsaw they have to wait until the visa centre in Moscow is open - well, wrong, it can be done by courier (or if they get their act together, directly at the embassy in Moscow). Entirely reasonable to ask them to accept the passport for stamping in Warsaw by courier (assuming decision made etc).