The btrfs manpage fails to document the
property subcommand, which I found by grep'ing the source. It's also in
To set a snapshot to read-write, you do something like this:
btrfs property set -ts /path/to/snapshot ro false
Change that to
true to set it to read-only.
You can also use
list to see the available properties:
btrfs property list -ts /path/to/snapshot
ro Set/get read-only flag of subvolume.
-t specifies the type of object to work on,
s means subvolume. Other options are
i (inode), and
d (device). If you don't specify, it'll show all applicable ones (for list) or try to guess for get/set.
Edit: in the newest btrfs tools, there is a
btrfs-property manpage documenting that subcommand, although it's not mentioned in the main manpage at all. It's also available as the btrfs-property page on the wiki.
(Note: This requires a new-enough btrfs-tools. Apparently on Debian Wheezy, you'll have to install the updated one from wheezy-backports; thanks artfulrobot).