I’m not saying that that wouldn’t work, but that seems like an excessively-complicated bit of lawyering.

If the goal is to provide NATO guarantees for part of Ukraine’s territory, but not to provide guarantees for another part of it, to counter Russia playing the “as long as I control part of your territory, you can’t join NATO” bit, the only thing that produces the guarantee is what’s on the paper of the NATO Treaty.

That treaty text is not written in stone. As long as all the members – and this assumes that we can avoid excessive shennanigans of the sort that Hungary and Turkey did around Sweden and Finland joining – are okay with it, the treaty text can be revised to say whatever. Yeah, you need unanimity for any such revision, but you need unanimity anyway to add a member, so the bar is no different from having Ukraine join in any other way.

NATO Treaty Article 6 defines the scope of Article 5 coverage.

https://www.nato.int/cps/en/natohq/official_texts_17120.htm

Article 6

For the purpose of Article 5, an armed attack on one or more of the Parties is deemed to include an armed attack:

• on the territory of any of the Parties in Europe or North America, on the Algerian Departments of France, on the territory of Turkey or on the Islands under the jurisdiction of any of the Parties in the North Atlantic area north of the Tropic of Cancer;
• on the forces, vessels, or aircraft of any of the Parties, when in or over these territories or any other area in Europe in which occupation forces of any of the Parties were stationed on the date when the Treaty entered into force or the Mediterranean Sea or the North Atlantic area north of the Tropic of Cancer.

In the original treaty, the bit about Turkey – much of Turkey’s territory is outside Europe – was not present. When Turkey joined, we did a small revision to extend NATO coverage – which originally did not cover territory outside of the Mediterranean, North Atlantic, Europe, and North America at all. Even today, the treaty does not guarantee against attacks on European territories like New Caledonia or American territories like Hawaii.

Honestly, I think that there may be a very legitimate argument that given that Romania and Bulgaria joined – and this becomes even more-significant with a Ukrainian membership – that the scope of Article 6 should be extended to the Black Sea, as we did with Turkey when Turkey joined. Otherwise, it’s possible for Russia to perform a blockade on NATO Black Sea powers and sink their warships without them being able to avail themselves of NATO Article 5 protection.