It all depends on the classification. The best comparison to the idea of viewing them as separate forms is Giratina-O, which could have been considered simply a set of Giratina, but instead it's considered a separate Pokemon locked to holding Griseous Orb. By this token, it's possible that any Absol holding Absolite could be classified as Mega Absol, a different form, which could be tiered entirely separately. This would mean that no matter how much Mega Absol might be used in the upper tiers, it wouldn't keep regular Absol out of the lower ones.
It could be a stretch, however, as the Mega Evolution doesn't change the form at the beginning of the battle, but instead is activated at will afterward. This would get especially weird for any abandoned Megas. If regular Gyarados is still used enough to make OU, but Mega Gyarados is completely overlooked in favor of it, then if "Gyarados holding Gyaradosite" is classified as its own form, it could mean that "form" would drop to UU. If people favor Gyarados's Flying typing and Intimidate that much, they could even end up using it in UU locked to Gyaradosite just because it's required to hold Gyaradosite to exist in that tier, never even bothering to Mega Evolve it. (No idea whether there would be any merit in keeping it as regular Gyarados if it's already given up the item slot, but let's speak hypothetically here.) This, I think we all can assume, would not be something Smogon would go for.
It's possible that we could see a more complex midway solution. For example, the Mega Evolution could be considered a separate form, but listed in the same tier as the original Pokemon by default, and only changed in tier if usage or brokenness of the Mega Evolution specifically would elevate it to a higher tier one way or the other. This is possible, but Smogon's tendency to seek simple bans means it's unlikely.
In the event that we don't see this complex middle ground solution, I would assume the items would just be treated like items like Soul Dew, and banned if they create a broken effect on the only Pokemon they're available to, but otherwise left as an option. This would mean that Pokemon like Kangaskhan, Banette, and Absol could disappear from the lower tiers, but it wouldn't be too different from Drizzle Politoed keeping non-Drizzle Politoed unavailable to the lower tiers in Gen 5 or Sticky Web Galvantula likely keeping non-Sticky Web Galvantula out of the lower tiers in Gen 6. When a Pokemon gets a better option that elevates them in tiers, we don't need to keep their original version available as an option in lower tiers; that would get ridiculous. Mega Evolution could be a special case, but does it need to be? I don't think it does.
