Of course, it would require support from the website owner but, in the case of popular sites like Gmail and Facebook for which thousands of user scripts already exist, the increase in customer satisfaction would surely be worth the minimal effort. Also this would only work on sites that require a login since you need to know who to serve which scripts to.
A quick search led me to Monkeygrease but that's not at all what I have in mind. They use a Java servelet to preprocess pages rendered by closed source web applications.
My suggestion is much simpler:
- Create an interface to allow users of the site to upload user scripts
- Store the scripts on the server
- Append the Greasemonkey environment and all installed user scripts to every HTML page served to the user
Is there a reason this doesn't already exist? Does anyone know of any projects going on in this area?