Debug registers are only accessible in Ring 0. I haven't looked at the implementation described, but I'd be surprised if it isn't a kernel module or something equivalent.

Are they available to a JTAG probe? I've not looked at a modern motherboard in ages, but I wouldn't be too surprised to find unpopulated JTAG pads near the CPU.

Only boundary scan on Intel.

I don't think x86 uses JTAG.

Why not? Even if it doesn't use JTAG for debug, it almost certainly would need it for boundary scan. I just checked volume 1 of the Intel i7 processor family datasheet, and it does show a JTAG port.

You are correct - it looks like it is distributed as a kernel patch.