[...] iTunes keeps its info in an encrypted database [...]
While it's been a while since I've used iTunes (like, since MC19, I think
), you are only partially correct. While iTunes does access and update data during runtime in a database with a proprietary format (which may or may not be encrypted), it also backs up all the data to a huge XML file in case the proprietary database becomes corrupt. In older versions of iTunes, that file was created or updated each time the application was shut down; however, since iTunes 12.2, it has become necessary to tell it you'd like to create that file. This option is available under iTunes->Preferences->Advanced, where you can check the box labeled "Share iTunes Library XML with other applications". You can then exit the application and the file will be created.
On a Mac, the XML file will be in the Music folder in your user home folder. Unfortunately, I'm not overly familiar with Mac storage anymore since my MacBook died a few years ago, so I can't tell you what the actual file path would be; however, once you find the actual location on the file in the GUI, it shouldn't be too hard to figure out the path.
On a Windows machine, the location of the file varies by Windows version. In Windows 10, it will be in "C:\Users\
your username\Music\iTunes". Previous versions of Windows may call the music folder "My Music", or even "My Documents\My Music", but wherever the Music folder is, the iTunes folder will be inside, and the library file will be within it.
Of course, that all assumes a default installation. If you moved your iTunes directory somewhere else using the iTunes option menu then YMMV. However, if you did that then you probably can figure out how to check your configuration to find the specified iTunes folder location, and the library XML file will be within.
Of course, that's only about 1/3 of the problem. The next hurdle is going to be the format of the XML file itself. Apple being Apple, they decided to totally ignore the entire point of XML and store their data as a dictionary of key/value pairs. You can see a sample of the format
here (or probably a hundred other places, that just happens to be the one for which I had a link handy. This is a bit of an excerpt here:
<dict>
<key>Track ID</key><integer>86</integer>
<key>Name</key><string>Play Your Part (Pt. 1)</string>
<key>Artist</key><string>Girl Talk</string>
<key>Album</key><string>Feed The Animals</string>
{other data omitted}
<key>Play Count</key><integer>1</integer>
<key>Play Date</key><integer>3381673926</integer>
<key>Play Date UTC</key><date>2011-02-27T21:52:06Z</date>
{blah blah blah}
</dict>
So anyway, it shouldn't be very hard to extract the data you want from the file. The hard part (at least as far as I'm concerned) is matching the iTunes data to a track in the JRiver database and then updating the value. Sadly, I haven't played with the JRiver API enough yet to know exactly how difficult that might be. Maybe somebody else can chime in with an answer here, or maybe I can go read some documentation and let you know. I just figured it might be useful to share what I know about the existence and location of the data first, in case someone else already had the third leg of the solution available. If there was, for instance, some kind of command-line method of doing it, then generating those command lines would be reasonably close to trivial.