@Nick you’re talking about absolute mouse emulation, right? AFAIK there are certain issues that won’t allow that kind of x/y coordinate transfpamation. While you can do so e.g. in ScummVM, because it’s only an interpreter, DOSBox emulates the whole mouse input.
What you can do to fix mouse pointer being off, is press the back button once the game has launched, then tap “Advanced settings”, then under Absolute mouse tap “Manual correction”, check “Enabled”, and play with the x/y resolution settings. This is particularly helpful in SVGA games and Windows.
In what game in particular is the mouse cursor off for you?