I think what you want is x264 ZONES.

https://en.wikibooks.org/wiki/MeGUI/x264_Settings#zones

Where you set an overall CRF normally, and then specify another CRF for a zone.

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Yes you can encode __MAIN using CRF and then the credits separately using 2pass, both with the --stitchable option, and join using MKVnix. Any acknowledgement would be purely up to the user. Or you can write a script to try & try again to match an overall size. I think this is retarded and not worth pursuing. After all, __MAIN is the bulk of your filesize, not the creds. Any dealbreaker for filesize typically would have long passed before the credits.

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1pass CRF is the highest quality mode x264 has to offer, better than 2pass, because the encoder is not concerned about filesize issues.

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you can use x262 for encoding MPEG2, and it apparently supports zones

https://www.videolan.org/developers/x262.html