Sorry! Although English is my first language, I don't seem to be able to speak it any more Here is my second attempt:
Rclone should allow devices, pipes and sockets to appear in listings (rather than give the error above) when using -M. This would mean syncs could transfer them and re-create them in the destination. I'm unsure whether you can sensibly back up pipes and sockets though.
Here's the thing though. I suppose that I could use -M to backup the 'pipes' but I think that I'd rather leave them alone whilst suppressing the 'notices' that rclone generates when (having been run without -M) it encounters such items. I take it that at present there is a way to do that, but not a fine-grained one - that sledgehammer-y way being to lower rclone's log-level.
I copied this behaviour from rsync which moans about stuff it can't transfer and expects you exclude it.
I think this log message comes before the filtering so you can't exclude it with a metadata filter yet, but nice idea.
I think that implementing the 'nice idea', and thereby being more rsync-y, would be the way to go. Still, I'd take either of your other options (viz.: a way of telling rclone to be quiet about, specifically, pipes.; and not reporting transferred pipes at log-level 'notice').