The pins aren't different, the "new" 1151 just has additional voltage pins. Signaling pins are identical. Coffee Lake engineering samples were used on Z270 motherboards. It has essentially been called a firmware lockout because lower end Z170/270 boards may fail to provide adequate power.
The "fewer reserved pins" are EXCLUSIVELY power delivery. Coffee Lake was used on older LGA 1151 sockets during development then locked out by firmware due to some motherboards being unable to provide sufficient power without those additional pins.
If the lockout is related to firmware and not signaling, it can absolutely be hacked to allow for compatibility.
That doesn't mean Coffee Lake will run properly on a $90 Z270 board - or any Z270 board for that katter. It only means that Coffee Lake's signaling is identical to all other LGA 1151 CPUs.