I use a VGA to HDMI dongle into a HDMI scaler with some vintage computers to make the signal compatible with the RM1V2 KVM.
This results in a 4:3 image centered in a 1080p 16:9 frame, and the mouse cursor ends up not aligning, as well as a forced border is displayed on either side of the image. It would be nice if there was an optional 4:3 Crop override added to the remote UI so that anyone else with a similar setup could avoid these issues too. Making it support an explicit X/Y coordinate & Width + Height would also allow doing region-of-interest cropping. Either / both of these could be implemented server or client side.
I suppose I could also replace the scaler with one that supports outputting more than just the 16:9 broadcast standards.
change EDID to 4:3 resolution make no effect?
Nope, the scaler (a Blackmagic Design UpDownCross) is a broadcast device with dip switches to select from preconfigured modes. The only 4:3 modes it will let you select are NTSC or PAL interlaced standard definition. Every other mode is either 1280x720 or 1920x1080 with varying frame rates and progressive / interlaced. Going straight from the VGA to HDMI adapter (j5 Create branded, with a separate USB connection for power & audio) has other issues, the colors are wrong, and modes such as 320x200, 720x400 etc are not accepted by the KVM. Inserting the scaler in the mix fixes the colors and allows many more of the low resolution modes to be accepted after scaling.