The suggestion about a transparent plane would work, but why not just insert a simple bit of 2D drawing right before the sync command. this is the way it's done in at least Unreal Tournament, because this is how you can change its RGB color.
for example
rem Cx1,2 and Cy1,2 are the vars for where you want your crosshairs
to appear
do
rem your game code
rem set draw color to whatever color you want your crosshairs to be
line Cx1,Cx2
line Cy1,Cy2
sync
loop
this way your engine doesn't have to declare yet one more object and you can make your crosshairs change colors at whim, by changeing a color variable.
