Note that I do NOT want to get rid of the X crosshair. My question is: How do I force my system to use that crosshair instead of a mouse??
In X, the cursor is set on a window-by-window basis. You can use
xsetroot to change the cursor on the root window (the background), but each application can override this. I'm not sure which cursor you mean by X shaped crosshair, but maybe
xsetroot -cursor_name X_cursor
does what you want? If not, here is a list of cursor names.
Also, the window manager (WM) can provide a default cursor, and other cursor shapes when it interacts with the application. For example, in
CursorStyle DEFAULT can be used to configure the cursor. As you didn't say which window manager you use, you'll have to look it up in the documentation of your WM.
You might find this related information useful.
Even old window managers can be tricked into using alpha cursors via Xresources (unless you've disabled this functionality in libX11 - it's built by default). When we first added libXcursor to Solaris about a decade ago, I tested 32-bit alpha cursors in CDE with no code changes with a simple:
% echo 'Xcursor.theme: whiteglass' >> ~/.Xresources
- man Xcursor
#ifdef USE_DYNAMIC_XCURSOR bitsin libX11 sources
Alan Coopersmith on TigerVNC Developer Discussion