720x400 70Hz is supported according to manual . That said, I cannot imagine how you can use an application like Fortemanager to adjust fullscreen DOS mode when you cannot see the Fortemanager application in DOS fullscreen mode.
The only potential workaround that I can imagine, is generating a custom windows display mode that matches DOS 720x400/640x400 70Hz timings and adjusting ForteManager to that .
The monitor might have nice display characteristics and having the option to control the monitor using an application is great, but having no physical monitor controls is a move so idiotic that it leads me to question the LG employed decision maker's fitness to hold whatever position he or she may have had . This my opinion .
That said, the fact that this did not catch on is a pretty valid indicator of this concept's stupidity, again, in my opinion .
I hope you can find a workaround or, if not, another monitor .