There's a way to do this with tf/resources/ui/HudAchievementTrackerItem.res and it's a lot better than cl_hud_minmode as it doesn't freeze up your game when you switch it and works just like you'd want it to.
Here are the steps:
1. Delete everything in your HudAchievementTrackerItem.res and replace it with this: http://pastebin.com/aDHB1nRp. Make sure it put your crosshair in the indicated section and delete the old crosshair in your hudlayout.res.
2. In your /tf/scripts/hudlayout.res ctrl+f for "HudAchievementTracker". Once you found it replace the entire thing titled "HudAchievementTracker" along with everything in the brackets with this: http://pastebin.com/ExSaAe3X
3. The trigger to turn the crosshair on is hud_achievement_tracker 1. To turn it off it's hud_achievement_tracker 0. Use a script that switches your weapons like mine for example: http://pastebin.com/KzR3yxCd
4. In game set an all-class achievement to get tracked so it can enable the achievement tracker.
I don't know how to remove the original hud crosshair so you'll have to use it too or cover it up. This is good for some people who want to add a ( ) kind of thing to they're normal + kind of crosshairs so you can toggle ( + ) and +.