I wrote:
There may be an additional wrinkle due to code and data portions of the distribution having separate version numbers.
Having reviewed the mailing list discussion around tzdb.tar.lz, I now understand that the era of desynchronised tzcode and tzdata releases is over, so this wrinkle doesn't apply to downloading current versions. For historical versions, I'm managing the split versioning by baking into my downloading code a table of all the historical releases since 1993 (including a couple that weren't announced on the mailing list; I think I got them all). So of the issues I discussed, the only one getting in the way of me using HTTP(S) is the lack of a machine-friendly way to determine the version number of the latest release. To reiterate:
The ideal would be to have an HTTP(S) URL that gives just the latest version number, as a plain text file in the same format as the "version" file in the distribution.
-zefram