I guess this heuristic should be very unlikely to fail with the current cbfstool implementation, so I'd be fine with it (at least as long as SeaBIOS is x86-only... non-x86 CBFS put the header close to the start address instead).
Still, is anyone except Chrome OS actually using CONFIG_CBFS_LOCATION and absolute offsets? Because we have completely switched over now, and the absolute offset was hacked in as part of our SeaBIOS build script which is now updated, so at least with our SDK you can't really build a new SeaBIOS with the old-style absolute offset unless you intentionally revert some stuff. If there's no other system out there that still relies on absolute offsets, I'd suggest to just switch over to the new relative style completely.