Is it grammatical to use ellipses to indicate a character reminiscing?

I find it quite acceptable (though 'acceptably punctuated' rather than 'grammatical' is the standard way to describe this on ELU), if it doesn't lead to non-negligible confusion. How many different ways is the overworked comma used, after all?

Finding a supporting 'authority' (there are no absolute authorities) proved easy. A caveat about cavalier usage was even included:

Ellipses ...

Informal writing

• In informal writing, an ellipsis can be used to represent a trailing off of thought.

Example

• If only she had ... Oh, it doesn’t matter now.

...

Like the exclamation point, the ellipsis is at risk of overuse.

[The Punctuation Guide]

Obviously, the trailing off of thought (into a reverie) can introduce a follow-up: here, a complete change of focus ('snapping out of it'), but in the original example, an adjustment of focus onto the reverie itself.

In the related Ellipsis in Fiction ... thread, OP states:

• I want to use the ellipsis to show a trailing of thoughts, not an omission/interruption.