We already have a similar request, MDEV-8473. But that entry describes two separate issues – a feature request for new options from 5.6, and an upstream bug https://bugs.mysql.com/bug.php?id=65812. So, I am going to convert this one, MDEV-8713, to a feature request, while MDEV-8473 will remain a bug report.
Please note that MDEV-8473 also requested --raw option.
I've added 10.1 to the list of 'Fix version(s)' just in case it can somehow be added there, although I highly doubt that, please don't see it as a promise.
Yes, thank you, the --raw option should also be included with this.
--raw
"By default, mysqlbinlog reads binary log files and writes events in text format. The --raw option tells mysqlbinlog to write them in their original binary format. Its use requires that --read-from-remote-server also be used because the files are requested from a server. mysqlbinlog writes one output file for each file read from the server. The --raw option can be used to make a backup of a server's binary log. With the --stop-never option, the backup is “live” because mysqlbinlog stays connected to the server. By default, output files are written in the current directory with the same names as the original log files. Output file names can be modified using the --result-file option. For more information, see Section 4.6.8.3, 'Using mysqlbinlog to Back Up Binary Log Files'. This option was added in MySQL 5.6.0."
https://dev.mysql.com/doc/refman/5.6/en/mysqlbinlog.html#option_mysqlbinlog_raw
Please note that semi sync was added to this feature in WebScaleSQL
Patch proposal:
http://lists.askmonty.org/pipermail/commits/2015-November/008692.html
Patch proposal v2:
http://lists.askmonty.org/pipermail/commits/2016-February/009051.html
ok to push
Are the only real changes here to mysqlbinlong.cc?
If so, can this be backported to a GA version, say 10.1, and/or even 10.0?
Many thanks for adding this, btw! 
Chris,
from the technical point of view yes, it can be backported to 10.0.