AntcuFaalb said:

5. I intend to mask the macroblocking by using a 120-frame Kodak 2383 35mm grain plate generated by poita. He says that this is likely to be the same film stock used for the release prints.

Have you tried any Avisynth filters for this? I read about one (or two) some time ago, but never had occasion to try it. A quick search brought up this (and there may be others of similar or superior effect):

Doom9 forum - Filter to remove macroblocks (from 2006)
http://forum.doom9.org/showthread.php?t=108511
Q: There's too much macroblocks in the source, those 16x16 blocks is
very visible and I would like to remove it ...
A: I'd recommend you to use DeBlock_QED_MT2() and if that doesn't remove all the blocking try this script: ...

Avisynth - Blockbuster
http://avisynth.org.ru/docs/english/externalfilters/blockbuster.htm
Blockbuster is an Avisynth filter designed to reduce or eliminate DCT blocks from an enocode. ... (Incidentally, DCT stands for "discrete cosine transform", and is one of the techniques MPEG uses to do its compression. They differ from macroblocks, which are most noticible in high-motion areas when the bitrate is insufficient to describe the motion accurately, and also from mosquito noise, which is an artifact that tends to appear around edges and areas of high contast.)