I know there are a lot of cvar configurations and people have found all kinds of tweaks for this game, but as far as I know, there is no way to configure hotkeys for weapon mods.
This would be a great addition to the game and I really hope it gets implimented at some point.. though I dont think it will be.
With the bow, you could change those arrow heads up on the fly. And maybe we wouldnt see so many laser scars running around all the time if you could swap scopes quickly.
The biggest problem for me when changing attachments is that if you moved your mouse around at all (ie. in the lobby, or with the TAB key held down) and then later on in the match, you want to quickly swap attachments and you have to find your cursor or move it across the screen.. delaying those precious seconds for hte kill.

The whole point of being able to modify weapon attachments in-game is so you can quickly change change your loadout to suit a specific situation, but if you can't do it within a few seconds, then you usually end up missing your opportunity and getting caught.

they are hotkey'd :/

x then the corresponding number.