Isn't this pretty much why language models were invented?

Pasting something directly into the chat interface seems weird, but if you could somehow just see where P(token | context) falls off a cliff, that's a pretty good hint that your writing has problem.

What would be a better way to incorporate AI as a spell checker?

In comparison to non-AI traditional tools, AI has the advantage of "understanding" the text, reducing the number of "stupid" mis-corrections. And its spelling correctness is usually already impeccable, so what is there to gain by interfacing it with traditional solutions, and how can it be achieved?

This type of laziness isn't novel.

Check out left pad or the two dozen other "utility" packages that could be done in a single line of code.

I swear that so many AI usecases I see are: "I did not have the skill or realize that you can write a program for this obvious logic".

I guess that works if you aren't a programmer or don't want to hire somebody, but then wtf would I pay for your service or product?