I have not tried this, but you should be able to play the track with output to Disk Writer and then you will have a file with just the part you want. It is somewhat laborious to do, but it might help for one or two files. If you use Audacity, you could slice out the part you want with that. Again, not a quick process for lots of files. In either case, you could import the shorter version into MC with a different name and use it as usual.
Yeah, it is certainly possible to export the file to an audio editor, trim the file, and put it back into the file system and then JRiver and any handheld will play the trimmed track. As you suggest, that's a lot more labor intensive than just putting a couple of numbers into a field in JRiver. It also has the possibility of overwriting data inside the audio file - and since I'm not sure which parameters are stored with the audio file and which are stored in some other database it's somewhat of a crapshoot. Hence the question in the original post above about what data is stored where.
A great feature for JRiver would be a "destructive editing" option for playback range that would do that in place and keep all the data intact. Maybe it does this already, but I haven't found it.
I've had to let go of a number of features moving from iTunes/IPod to JRiver/Android, but the ability to easily top and tail tracks and have the handheld use that info is the one I miss more than any of the others. So far, the increased flexibility of JRiver and the improved sound quality from both JRiver and the Sony player (with it's own DAC chips) makes it worth it. And of course iTunes/IPod is deadware, so there's that too.
Long term, I'm probably going to get a windows machine for the condo and "clone" JRiver so I have the full features at both places, but it would be nice to get the top and tail functionality on the handheld.