The nice thing about having the check procedure be part of the <file-system> is that it can be overridden. I'm not sure what use cases there are yet. One I can think of is that btrfs device scan is only required when using a multi-device configuration like raid. So I don't know if we want to run it by default as a %btrfs-checker or do nothing by default, and let the user add a custom file system pre-mount hook (file-system-checker) when needed.
A version of Marius'es patch that works would be here: http://lists.gnu.org/archive/html/guix-devel/2016-12/msg00087.html
