a lot of distros split these now into seperate packages. may be worth looking at some of their methods, particularly there method of handling dependancies between the two.

if its just the files in zoneinfo, would it be easier to build the new files as mentioned above to a temp directory, to integrate into new package. Extract libc6 to remove old zoneinfo files, edit config and rules etc, then rebuild package, keeping binaries intact.

not sure if divert could help as a solution for non cssu.