Umodel now has GUI
I know that nobody expected that from me, but I decided that at this point UI is required feature for UE Viewer (it will be highly required for Unreal engine 4 support). So, I spent 2 weeks of nearly full working time (with a cost of half of my salary) and created my own UI framework, and then used it for making a basic UI for umodel. This is still work in progress, so don't expect much today, but expect more in the nearest future.
Umodel picks up command line options and fills corresponding UI controls. Also, you may start umodel without a command line, in this case "Umodel startup options" dialog window appears. After selection of startup options, another dialog will appear, where you may choose a package to open or export. You may open that window at any time by pressing <O> key.
Please note that UI is still under heavy development, so probably you'll find bugs or missing features.