Why not handle it the same way they handled the stale udev API -- twice? Just create a kconfig option for legacy-powertop-compatible event headers. New distros using the new powertop can simply turn it off, as can users building their own kernel who either don't use powertop or use a new enough version, while the distros and users building new kernels for an old distro install can turn it on if they need to.
Then, after some time (preferably somewhat longer than the 3.6/3.7 timeframe mentioned in TFA, we're already in the 3.5 cycle, after all, and 3.7 could well be before year-end), that option could disappear. But meanwhile, only folks unwilling to upgrade what was after all a 2.0-pre-release powertop to the full 2.0+, would have to suffer the additional overhead.