I have an old HD11a which is just a re-branded CM11a, and I verified that it does work through the usb/serial converter I have.
Using optware ipkg (ipkg is a must!) I was able to install HEYU. They also have one called Ppower or Penguin Power, which I assume would also work:
DiskStation> ipkg list | grep "home automation"
heyu - 2.8.0-1 - X10 home automation control using the CM11A
ppower - 0.1.5-1 - Ppower, short for 'Penguin Power', is a piece of software for controlling x10 home automation equipment connected to the comput
domus.Link PHP frontend for HEYU
I wasn’t too excited about cgi/perl scripts, but I found what I think is one of the only PHP front ends which runs perfectly on the Syno after slight tweaking of the php.ini file.
I was even able to create a 3rd-Party applications node for it to run embedded in the web manager.
DiskStation> ls -laR domusLink/
drwxr-xr-x 2 root root 4096 Dec 23 00:53 .
drwxr-xr-x 3 root root 4096 Dec 23 00:33 ..
-rw-r--r-- 1 root root 198 Dec 23 01:01 application.cfg
lrwxrwxrwx 1 root root 44 Dec 23 00:53 images -> /volume1/web/domus.Link/theme/default/images
DiskStation> cat application.cfg
text_enu = domus.Link
description_enu = domus.Link - Home Automation Interface
type = embedded
protocol = https
port = 443
path = /domus.Link
icon_16 = images/menu_home_on.png
icon_32 = images/menu_home_on.png