Editing scrollback is possible in eshell, and something I use often.
Anyway, unfortunately that is not possible in a Ghostel buffer and most likely also will never be. I'm open to ideas though how we could improve or replicate your eshell workflow.
But also, eshell is awesome and Ghostel is not a replacement for it. It's more a replacement for term.el, maybe shell.el (with line-mode) and other terminal packages like eat and vterm.
Ghostel looks really nice though!
I didn't even think of the use-case that then you can edit output from finished commands in the scrollback. But it makes sense and is another +1 for that feature request.