Basically, no. A deliberate design decision was made early in Perspective’s development to not allow arbitrary Javascript to be written by someone in the designer - which would be your way to access “internals” like you can in Java Swing (caveat: not really, because React is in the way, but basically).
On the plus side, adding an external component to Perspective is, generally, easier than Vision - personally, I’d like to add some kind of easy builder that handles the (minimal) Java portions required and makes it easier for front-end-ey folks to just supply their own JS code much more easily.