Inspired by the latest RB TAS, I tried to work out how you would get a legit GSC complete Pokedex (minus Mew and Celebi).
The exclusives that I found you need the 1st generation games would be: Bulbasaur line, Squirtle line, Charmander line, Hitmonlee, Hitmonchan, Omanyte line, Kabuto line, Porygon, Articuno, Zapdos, Moltres, Mewtwo. Hitmontop and Tyrogue need to be bred from the Hitmons too.
So, in theory, you could add a single 1st gen game to Gold and Silver, but it would require you to reset 3 times, at least for the starters, three of the Eevees needed (probably faster than breeding since they take a long time to hatch), the Hitmons, and the fossils. I think this would differentiate it from the R/B TAS, given that you're not essentially duplicating the effor there in addition to supporting the completion of the G/S dex.
Some breeding is needed anyway since the baby level Pokemon exist, and evolution stones and held items like Metal Coat are extremely limited too, so planning would be a difficult task, but I'm sure it would be possible.
Just like the R/B TAS, the end point would be getting the certificate after getting all 249 Pokemon in both Gold and Silver.
So, my basic thinking might be this in planning:
Red or Blue version:
1st: Trade starter early on to Silver
2nd: 2nd starter, rush as fast as possible to Cinnabar Island with one fossil, grab one Hitmon, one Eevee
3rd: 3rd starter, play through to beat E4, obtain Mewtwo, second Eevee, other Hitmon, other fossil (+Aerodactyl)
Gold will be used to obtain the extra starter, means Silver can do some more time consuming tasks like levelling Larvitar.
Of course, this is all from a basic overview of what is possible, I'm not considering any problems from syncing the versions for trade or planning for which Pokemon to get in each version. A more detailed plan might thrash out a lot of what each specific version needs to do in each cycle, but some of that may be up to the RNG. Still, it is something I'd like to see, and I'm sure I am not the only one.
