Similar to Ring of Elysium's team marking system. You would be able to aim and hit a keybind allowing a transparent mark that would be shown to you and your teammates at that location where the mark is able to be seen through walls and such, the mark would fade in and out with a noticeable frequency. After the mark would disappear over a short delay.
If this turns out to be possible, adding the feature of selecting the color of the mark you show your teammates could help distinguish each other as well.
I am ignorant in the abilities of the Source engine, but hopefully someone out there is willing and can do this.
Also i''m will pay 10$ to anyone who can do this for me.
(You can buy a lot of jelly beans with 10$)