So I think I've got an idea of what might be going on here. For OP, MC doesn't implement it's own file browser for Linux, it just uses the system file picker. I'm on Gnome not on KDE, and when I try to browse files in MC it uses the GTK file picker that most other programs use on my system when browsing for files (firefox and Calibre both use the same GTK file picker for example). When I mount a network location using my file manager (not via fstab) the network location appears in the file manager (files or nautilus), but not in the GTK file picker that almost all other programs use on Gnome. So for me MC uses the GTK file picker, and I see no network locations, but that's also true for most other programs I use on Gnome (i.e. firefox and calibre both also do not see the network locations because they're using the same filepicker). VLC is an exception, it seems to use the QT file picker that KDE uses rather than the GTK file picker, which makes sense as I think it's a QT-based program.
Here's what I wonder: obviously KDE has it's own QT-based filepicker that, based on OP's report, other apps on KDE are using (i.e. OP sees network locations in other apps.). Is the issue just that MC is using the GTK file picker rather than the QT one on OP's system? OP, does your MC file browse window look like the attached image (or a white background version with the same layout)?
If so it's not really an issue that JRiver can fix exactly, the GTK file picker is a general Linux file picker that many, many Linux programs use. It might be possible, if the devs wanted to, to support multiple file pickers (I've seen some programs that seem to use different pickers in Gnome and KDE), but that seems like a fair bit of work.
TL;DR- I suspect that the issue may not be anything the JRiver devs have done or can do; they may have just picked one of the two most widely used file picker frameworks and the one they picked happens to not include network locations at the moment. Of course I may be barking up the wrong tree, but I had a hunch.