I guess you could acheive that effect by using the 'quickedit' function of an MSdos shortcut, but it would be ugly, and, to be honest, unnecessary....
If you just toy around with the program on the first level, you'll quickly see that it follows the mouse pointer just fine. I can understand that it would be nice to see the whole of the target square lit up (particularly considering fullstops being 'the floor' in doomrl, and thus rather off-center)... However, I can't think of a good way of having the target square highlighted without detracting from the 'one click, one shot' functionality of the program.
Perhaps if a left-click displayed the target square, and a right-click fired at said square? Would that suffice?
Edit - thinking further, I guess the program could detect when a user presses the 'f' key, and when they do, makes the 'X' follow the mouse pointer, until the user fires (with f, space, or mouse 1), or cancels (escape). If you want this, I'll tinker with it on monday and try to get it out.