Edit: see ripsaw's post below for information on disabling the built-in mouse driver. Note this is not recommended for general use.
With the built-in driver disabled, I was able to load a Microsoft DOS mouse driver, such as "Microsoft Mousedriver V7.04 for DOS" (like so at the commandline: mouse.com /P4). The setting /P4 may sufficiently lessen the acceleration. It could be compared with running the built-in mouse driver with a low or default sensitivity setting. Edit: as noted below, the native mouse already has a flat response.
This mouse.com version should support many of these parameters:
Setting Default Values Switches
Sensitivity
Horizontal 50 5-100 /Hn
Vertical 50 5-100 /Vn
Hor & Ver 50 5-100 /Sn
Acceleration 2 1,2,3,4 /Pn
Hardware Cursor
Support off on/off /Y