Dear Adam,
probably a better forum for this questions is the LinkedIn “SDTM Experts” forum (LinkedIn Login, Sign in | LinkedIn) as all the SDTM gurus are on that one. So you might also want to post it there too.
Regarding the matter itsefl, I would also prefer option 2.
In principle, SDTM should not be about derived data, but unfortunately the model deteriorated over time “for ease of review”. Baseline flags should in principle not be in SDTM, but reviewers wanted them, so they were added. As such, I see no harm in putting the average as a separate record, and flag it with “baseline”. It is not because it is not on the CRF that it cannot appear in SDTM. There are many other examples (such as the --DY variables) that do not appear on the CRF but that are in SDTM anyway.
Best regards,
Jozef