First, to satisfy CA rules of professional responsibility, I must say that I am not your attorney, nor am I looking to establish an attorney-client relationship with the original poster or anyone else reading this thread. If you want legal advice, I suggest that you seek out a competent attorney who can counsel you on such matters.
That said, I like custom guns. A lot.
One of my CCW guns is a Gemini Customs S&W 640 .357 j-frame. Another is a Colt Defender which has been extensively modified by Springfield Custom. Soon I'll also carry a stainless Colt Commander, even more extensively modified by Springfield Custom.
I have given this topic some though, although I must admit that I have not searched for case law or otherwise given it any serious legal research. Here is my opinion (not to be taken as legal advice) -
1. In my opinion, one should get any and all custom work done by a well-known, established and accomplished gunsmith who specializes in that particular platform. Mine have all been done by members of the American Pistolsmiths Guild, and my 1911's are worked on by a winner of the APG Pistolsmith of the Year winner. They are well known for their expertise and quality of work.
2. Keep receipts, build sheets, everything. That way, the gunsmith can verify the work he's done on your gun.
3. Take your custom guns in for safety inspection. Do not modify them after that point.
4. Be prepared to explain why do had the work done. Take for example, the Hydra-Porting on my custom revolver. It reduces recoil, which allows me to shoot more accurately, and hit only what I intend to hit. The trigger work the guns? Same reason. All have passed OCSD safety inspection with their current, modified triggers. Refinish? Hell, I just look like the way it looks. Carry bevel? Less wear and tear on holsters, more comfortable to hold.
I would NOT carry a gun that I modified myself, nor would I carry something modified by "a buddy", a local guy who is a small operation, etc. The names of the people who work on my guns are nationally known and respected in their industry. If something ever happens, they will most likely still be around, respected, building custom guns.
I would NOT carry a 1911 with Wicked Grips grim reaper medallion grips, Punisher grips, "Kill em all" grips, or the Glocks you see with custom engraving on the muzzle that reads "smile, wait for flash" or whatever nonsense people do.
New safeties, trigger jobs, etc. are okay, but I would never remove a safety feature. For example, you can remove the Series 80 system from a new Colt quite easily with parts from Brownell's. I would never do this on a carry gun... hell, I wouldn't do it on any gun, because you never know when a gun might need to be used defensively.
Paranoid? I don't think so. Remember..... limit liability and CYA.
Oh, and your safety inspection sheets that you get from the armory? Keep a copy on file as long as you keep that gun on your CCW. But, I think that's a good idea for any gun on your CCW, modified or not.
Finally, when I write up a build sheet to get a custom gun built that I intend to use for carry, I clearly express on the build sheet that the gun is for CCW use and must be safety inspected by my issuing agency.