DWF, I concocted the same idea years ago concerning the Doctor's programming of Xoanon which resulted in the cultural split. And, as far as I can remember, independent of interaction with other fans. I thought, "Why can't the Doctor remember his first visit, especially since he had companions travel with him from the start? What would make him forget?" Then I thought about about Davison's post regenerative trauma, being "out of it" for a couple of episodes. Hey! That might be the ticket! Baker's Doctor was still "rattled" from his recent regeneration. It would even explain why Xoanon developed its "mania". But when could have the Doctor left to perform those actions, still partially delirious from regeneration? Why, the scene when Sarah and the Brigadier find Harry Sullivan "truseed up like a pair of old boots". In fact, we even heard the "groaning" of the TARDIS' engines. "But it never dematerialized," some will counter. From our perspective, maybe not, but who's to say the TARDIS didn't start to dematerialize and then, mere pico-seconds "out of phase", the Doctor is returning from that adventure?
However, I will admit there is a fundimental flaw with that notion. The cliff-side portrait of the Doctor seems to sport his iconic scarf. Yet, when the Doctor opens the police box door, responding to sarah's pleas, he's still wearing the nightshirt when he trussed up Harry.
A less intriguing notion is that the Doctor's first encounter with Xoanon was simply an "unseen" adventure someone after the events of "Revenge of the Cybermen" when the TARDIS finally caught up with our trio. (As presented, there was really no point during Baker's first series when he could have traveled to the Sevateen planet. The dialog omplies the gang "vworp" immediately at the end of "Robot" to the Nerva station in "Ark in Space". At its conclusion they teleport to earth in the "Sontaran Experiment". From that point, their teleport beam is intercepted and redirected by the CIA and aimed at Skaro for the "Genesis of the Daleks". And using the time ring given the Doctor, the troupe make their way back to Nerva to experience the "Revenge of the Cybermen". Even at the end of that story, the dialog suggests the Doctor, sarah and Harry head straight back to "present day" Earth responding to a space/time telegraph message the Brigadier transmitted. After point, the field is "wide open".
I just like the idea of the Doctor having an entire adventure in the "blink of an eye", so quick it doesn't even appear he left.
Sincerely,
Bill