The best CPU ID to use for the Coffee Lake I7-8700K would be that of the I7-7700K. We'll find out for certain once more people complete and test builds.
The reason that Intel didn't allow the use of Coffee Lake chips on Z170/270 is primarily power delivery and overclocking issues. If you overclock a 6 core 8700K on an older 170/270 motherboard it would likely cause many problems. Here's part of a review from Tom's Hardware.
"We know that the existing Z270 motherboards can provide enough power to push quad-core processors, as we see now with the Coffee Lake Core i3 processors, but Intel noted that the additional two cores proffered on the i5 and i7 would require more power."
"Although TDP isn't a direct measurement of power consumption, it is a decent indicator. The Coffee Lake i7-8700K weighs in with a TDP of 95W compared to Kaby Lake i7-7700K's 91W rating. A small increase, sure, but we could see larger deltas during overclocking. Intel says it improved the package power delivery to offset the increased overclocking power requirements for the six-core models, and we will certainly quantify the difference in package power draw during our review. The Coffee Lake processors also support per-core overclocking, a feature that wasn't included in the Kaby Lake era, but they still don't allow for fine-grained per-core voltage or P-State settings."
So Intel and mobo makers know that if they allowed 8th Gen CPU owners to use the older boards because there would be some "enthusiasts" doing major damage to their Z170 and Z270 systems. They can't allow just Coffee lake 4 core chips or even the six core non K chips to be installed on the older chipset motherboards and disallow the 6 core K versions at the same time.
Most motherboards have 3 year warranties. All Kaby Lake motherboards are still within that warranty period.
Here's another interesting point made by the Tom's Hardware article.
"Kaby Lake processors will also not work on 300-series motherboards. Intel indicated the decision to eliminate Kaby Lake compatibility was due, at least in part, to requests from motherboard vendors that the company make a "clean split." For motherboard vendors, this removes the burden of adding support for Kaby Lake (and the requisite validation) during a time when most motherboard vendors are already stretched to their engineering resource limits due to rapid fire Intel and AMD launches."