S1 and S3 are implemented in the 30Jun10 patch.
S2 is not. Dump files do allow you to append (see dump_modify append).
But they write snapshots which are self-contained. The various
fixes (like fix ave/time) write headers followed by lines of info. If you
just appended, the header would appear again, which would mess up
the post-processing of the file. So I think it's cleaner to write new files
and force the user to think about how to merge/combine the info.
Steve