Yes, I realize that it's manually reinventing the wheel. However the context here was supporting random access (and append, truncate, etc) while also using sepa

Yes, I realize that it's manually reinventing the wheel. However the context here was supporting random access (and append, truncate, etc) while also using separate formats for archiving and compression. Such an approach is more work but provides clean separation and thus far more flexibility. Bear in mind that the alternative (in context) is throwing a bunch of independently compressed files at something like tar. In the event that the files are small, that will absolutely destroy the compression ratio. If the files are large enough or you only have a few then you could always omit the custom dictionary.