Is it feasible for zic to encode variable information in the data file for the last set of rules, that would be used for times past the last entry in the transition table? Then localtime() would use the variables algorithmically rather than using table-driven data, for dates past the last table entry. Perhaps set a minimum size for the table entries (ie, have table entries at least up to year xxxx.)
This approach isn't as general as the extend-400-years approach. Take, for example, the proposal a few years back to extend DST into November in the Pacific Time Zone in presidential election years (as a way of minimizing announced results from New York affecting voting in California). The irregularities that would have resulted can't be handled by a POSIX-style rule string; they can be handled by the extend-400-years approach.
I'm not sure yet, but with 9x larger timezone binaries, Sun might retain the old-style timezone binaries for timezone patch updates. If so, it might be useful if zic has an option to produce old-style timezone binaries.
That's certainly doable. Note, though, that the new binary files start with information in the old format, and should be usable on old systems. --ado