Absolutely. If your account has access to the data, you can basically rewrite a website using a bookmarklet.

I wanted a friends feed in eBird (its social model is inscrutable) and ask Claude to make a bookmarklet that showed the most recent lists of my friends sorted chronologically. One shot and done.

Our internal corporate CRM is siloed data queries spread across overlapping views. I have a bookmarklet overlay my own interface on top and fetch data using the existing session token. (which is transparent/automatic - your browser just hangs on to the session cookies for the fetch calls)