While an icon has been added to midnight commander. Here is an explanation of how an icon can be added to an application, in case someone wants to add an icon to another app.
To make a new extension you need the mksquashfs extension installed.
You need a suitable image for the extension. Look upstream, there may be one available. If not, look at the images of the other icons, and make something compatible. You can find these in tmp/tc/loop/(app-name)/usr/local/share/pixmaps/. (Apps need to be mounted to access them. If they are in OnDemand, start them and then close them.)
You need to add or modify the .desktop file. Look at the desktop files of other apps, and make something similar. These can be seen in tmp/tc/loop/(app-name)/usr/local/share/applications/. If your app already has a .desktop file, you may only need to add the line about the icon image.
When you have an suitable image file, and a suitable .desktop file you can make the new extension.
You need to have your app mounted. If it is in OnDemand, start it, then close it.
Using a file manager, create a new directory in /home/tc. Let's call it myext.
Create a new directory in /home/tc/myext. Use the name of the app. Let's call it app-name.
Go to tmp/tcloop/app-name. Copy the contents, normally a directory usr. In some cases there may also be other directories. Paste this to /home/tc/myext/app-name.
Go to /home/tc/myext/app-name/usr/local/share. Create two new directories. One called pixmaps, the other called applications.
Copy and paste the icon image to pixmaps. Copy the app-name.desktop file to applications.
Open the terminal and type:
sudo su
cd /home/tc/myext
mksquashfs app-name/ app-name.tcz
Using a file manager, remove the original app from /tce/optional. Copy and paste the new app to /tce/optional.
Remove the app from OnBoot or OnDemand.
Reboot the computer.
Add the app to OnBoot or OnDemand.