Thanks for bringing these issues to everyones attention.
I compared One Shot Mode, against the new 1DX AI Servo III and the 1DMKIV AI Servo II using the Canon 85mm f/1.8 set to the standard default Case 1 tracking. I was able to replicate the lack of AF in AI Servo in very poor light, with a low contrast stationary subject. The way I was able to simulate this test was by hanging a gray towel over the shower curtain bar in the bathroom. The test was performed during the day, so I was getting some ambient light coming down the hallway since there was a window about 15 ft away. I left the light off in the bathroom and then I stood just outside of the bathroom doorway. So, I was actually just standing in the hallway.
With the 1DX, I took a shot at the center of the gray towel in One Shot and the camera was able to achieve focus lock and the ISO was 25,600. I then took a shot of the towel in AI Servo and the 1DX wouldn't AF on the towel. The shot was blurry and out of focus. I also tried to shoot the edge of the towel in AI Servo, but the camera still wouldn't Auto-focus.
I then repeated the test exactly the same way with the 1DMKIV and I got the same exact results. In One Shot the MKIV locked in on the gray towel and it was clearly in focus, despite giving up an approximate 1-stop advantage to the 1DX. Then I switched to AI Servo for the MKIV and the camera would not AF on the towel and the shot was definitely blurry.
So, it seems to me that this is not necessarily an issue isolated to the 1DX only. One thing that I would note is that I would never use the camera this way, under these conditions. If I were taking a static shot like this I would switch to One Shot, which can easily be added to the DOF or Multi-2 button. Since AI Servo is optimized for movment and tracking, perhaps Canon has made a trade-off here and they have sacrificed AI Servo sensitivity for static subjects in "low light with "low contrast." Perhaps movement or motion of the subject would further assist the performance of the 1DX and 1DMKIV in AI Servo? I would also like to add that I have never had any issues tracking athletes in dark uniforms in poorly lit stadiums with the MKIV at ISO 12,800. I have not tried the 1DX yet under these conditions.
I think it would be rare that I would be shooting "static subjects" under such "poor lighting", with such "low contrast" while using AI Servo. However, if others find themselves in this situation, then I would switch to One Shot Mode. AI Servo and One Shot have been optimized for their strengths and weaknesses in individual shooting situations, if they weren't then we would only have one choice. The 7D has AI Focus which is a hybrid mode between One Shot and AI Servo, if your subject was standing still then it would shoot in One Shot mode, if your subject moved then it would switch to AI Servo mode, but it wasn't as good as either mode used separately, so that is probably why Canon has not incorporated AI Focus mode into the 1D series.
I noted that some users above were also trying to track a low contrast subject walking around the house in poor lighting, also noted was the blurry wedding photograph which did have some contrast, and as far as I'm concerned is completely unaccepatble in decent lighting like that.
Would I like this to not be an issue? Of course! Would it be nice if we could have AI Servo perform better under these condtions? Absolutely!
If Canon could improve this by increasing the sensitivity of AI Servo at an ISO of 25,600 that would be cool too, however the downside may be that AI Servo would become very skittish and cause the AF to jump around with the slightest movement of the subject, which may then have other ramifications. As the OP and Canon has mentioned, perhaps some tweaking of the custom AF cases and sensitivities will help improve this situation.