I am looking for something that functions to what amounts to JavaScript injectection, but safer. This idea can be most effectively observed by your broswer's console and start noodling with something like document.getElementById('btnSubmit').value = 'I took over your button'; after a quick inspection of the site in question. I would effectively would like to amend an established website to a local machine for the purpose of pre-filling fields as an exercise of efficiency in a work process.
The Background
So far, JavaScript seems to be the best candidate for accessing elements with a document read in by a browser, and JavaScript injection is the only feasible method of sort of accomplishing this idea, but that's just from my limited perspective. Many of the processes in question requires duplicate data input in more than one system where at least one of the data input sources is at least controllable by a proprietary server-side language. I am open to suggestion, however, I think the question itself is interesting.