By delving in to what remains of the competitive Tribes: Ascend community I have discovered their preferred method of using custom crosshairs. Instead of customising their HUDs as we do, they run their game in a 1920x1080 borderless window and run an AutoHotkey script to force a PNG image onto the top layer of their screens (therefore above the game window). This allows the crosshairs to be used in any FPS game that can be ran in a 1920x1080 borderless window! The script itself allows for a square PNG image to be automatically centered. Using a free online image editor I was able to create a number of sample crosshairs for you to download to try out (A little over 50).
I have also included some of TF2's default crosshairs to be used as a way to bypass crosshair animations such as when using the Ambassador.
Images of my crosshairs Spannzer's Crosshair Pack v3
Download link to my crosshair pack (Zip folder) Includes all of Spannzer's Crosshairs v3 and a few bases to start making your own.
A link to my copy of the AutoHotkey script EDIT: The zip folder now has everything inside it, Dropbox uploaded an earlier version of the zip folder for whatever reason.
Someone else's instructions on how to get TF2 to run in a fullscreen borderless window
Note: Many of the crosshairs I've made are based heavily on other people's crosshairs from a multitude of HUDs I've used over the years.
The crosshairs I've made are made using a 600x600 transparent square with a perfectly centered 2x2 pixels square as a starting point, I encourage you to do the same by editing my crosshairs with image editors yourself to change things like colours or the actual crosshairs themselves to your liking. Remember to save the images as PNG files to allow the space of the 600x600 window not taken up by the crosshair to be transparent. I myself have not yet tried making crosshairs in windows larger/smaller than a 600x600 window, so I can't comment on it.
The crosshair script itself (obviously) is not affected by sv_pure since it's basically a more advanced way of sticking coloured tape to the middle of your screen.
The script allows for two active crosshairs that are toggled by the "END" key. (Press end to cycle between your first crosshair, then your alternate crosshair, then again to make the crosshair invisible)
The readme file in the folder the script comes in explains how to install AutoHotkey and change the toggle button and things like that.
To change what your two active crosshairs are, rename a crosshair's file name to either: Crosshair or Alternate and move them to the same folder as xhair2
I hope others find this as useful as I have, I've tried my best to cover any questions that may be raised by readers.
EDIT: Some people are experiencing FPS drops while using this, if you are karppoh has disclosed on this thread that this is due to Windows Aero forcing v-sync on while running TF2 in Windowed mode. To disable Windows Aero follow these instructions. I personally have reduced FPS while not running Aero and running the crosshair script however. Experiment with both if you are having FPS issues.
You can use a free online image editor such as Pixlr to edit the crosshairs' colours and whatnot. Make sure to save it as a PNG.