The above file might be a solution. I only just made it this minute, so haven't had time to test it thoroughly yet, but on first sight it seems to be working as expected in P6. I suspect it will work in later versions as well.

Just a cross-hair prop, smart-parented to the FocusDistanceControl. The prop is locked by default to stop it being accidentally moved out of alignment, but can be unlocked if you want to scale it up or down. Changing its Element Display style can give some variation in the thickness and colour of the lines, the Foreground Color can also have an effect in some display styles. The props material is set to transparent, so it should not show up in a render, or just hide it if you no longer want to see it.

Save it to any props pallet, and loose the ".TXT" part of the file extension.