Isn't that exactly what you were asking for earlier in the post?
Unless it's something like Armor Lock - i.e. there is literally no skill involved in using it, whatsoever - this won't be a problem. If someone picks it and doesn't use it properly, then that's no different from getting hold of the Sniper Rifle and missing every shot, or sprinting up to someone with an Energy Sword. If someone does use it properly, what's your problem? It's no worse for you than if they're really good with the Battle Rifle.
As for Custom Loadouts, I can see them being a lot of fun in social matches and Custom Games, especially if individual loadouts can have their physical traits modified as well (e.g. Health, Shields, Speed, Motion Tracker, etc.), or for 'Elimination' type game modes. I'm not so keen on them being part of ranked game modes, but I'm pretty sure it could be done without breaking the game. Besides, if every non-power weapon is properly balanced, does it really matter what someone spawns with?
Contrary to popular belief, everyone spawning with the same weapon doesn't actually make for perfectly balanced gameplay. What if everyone on a team was great with the Assault Rifle, but bad with the Battle Rifle? In a Slayer match they'd dominate the opposing team, but get hammered in a Slayer BR game. Two completely different outcomes, but both from an apparently 'fair' starting situation?
If everyone starts with their preferred starting weapon, then the best team will win and it won't come down to you not getting the gametype variant that you're best at. This is of course highly dependant on getting the initial spawn weaponry balanced, but if they are then it actually makes the game more balanced, more fair, and more competitive.