Do you need one shot?
I know we are all used to using it. But with good quality AF points, many people use AI servo even for things like portraits.
If you are shooting someting like the 85 1.2 wide open for a portrait, for example, the depth of field might me 3-4 inces.
Between you swaying a bit and your subject swaying a bit, that can put teh focus on the ear instead of the eye (I know I miss focus almost 50% of teh time in fashion trying to shoot that way.)
Given all of the tunable parameters and points for servo, maybe that is the better overall choice? Allow it to lock and maintain focus. Then switch bewteen spot,, expanded, tracking, etc. depening on what you shoot?
You can also change the priorities too. Focus as highest priority for portraits, others for sports, etc.
I believe that the AI Servo parameters (tracking speed, focus priority, etc.) can be registered, but not the AF mode itself - that seems to be what the manual indicates. It states, "Convenient when you want to change the AF characteristics duringAI Servo AF, not to AI Servo AF. However, I haven't tried it yet.
Ok, I just tried it, and no, Switch to Registered AF Function does not change you from One Shot to AI Servo.
Still, that combination combined with the one-button press-and-hold change to AI Servo would do it, i.e. assign the registered AF function to the DoF Preview button and use that for a one-touch change from single AF point selection to 61-point auto selection, and then press and hold the M.Fn2 button to change from One Shot to Servo. Obviously, that's only possible on the 1D X with the M.Fn2 button right next to the DOF Preview button.
Still, I currently have C2 assigned as One Shot with single point selection, and C3 as AI Servo with 61-point selection, and I can rapidly switch since I have the M.Fn1 button assigned to Switch to Custom Shooting Mode. That's better for me, because in Servo mode I also want a higher minimum shutter speed setting.