The default config.xml file has the following section available indicating that this will enable subtitles:
Uncomment the lines below to get rid of jerky avi playback on the
DSM320 or to enable subtitles support on the DSM units
<add header="X-User-Agent: redsonic"/>
I have a DSM-520 running current firmware version 1.06 and can confirm that the following line must be changed to the following in order for subtitles to work. It must have the "http://www." prefix in order for subtitles to work. This should likely be changed in the default config.xml file.