Relative mouse support?

We have a system that is using a custom OS, I've seen it mentioned in a few threads that only absolute mouse input is currently supported. Left and right click works, but the mouse cursor is stubbornly stuck.

I've seen emulators and remote desktop viewers that have a mode switch. I realize that capturing the mouse in a browser window is tricky, but the OS has its own mouse cursor, so if there was a way to send relative moves based on the mouse movement in the host window, that would make the system usable for us. Alternatively, UI mouse move arrow buttons would be clunky but better than nothing.