This wilful insanity is completely incomprehensible to me (HTMX and lit are also fully infected with it): "Oh no, we are just HTML, we can't do anything" while literally doing tons of things outside of HTML.
Again: you literally have a custom Javascript-like DSL in Datastar. Use that.
It's like the "it's just HTML" or "it's HTML-compliant" mantra somehow damages the brain, or something.
Edit: this custom JS-like DSL is so prominent and such a crucial part of Datastar, that it's referenced in the very first paragraph of reference: https://data-star.dev/reference
Oh, look, your HTML-spec-compliant thing in which it is apparently impossible to do anything outside data-* attributes somehow calls external functions, and updates signals, and reads signals, and does all sorts of things:
<div data-signals-result>
<input data-bind-foo
data-on-input="myfunction(el, $foo)"
data-on-mycustomevent__window="$result = evt.detail.value"
>
<span data-text="$result"></span>
</div>