In these days IDEs like Eclipse understand Java (for example, it's also true for some other popular languages) better than text editors and they are able to provide features like intelligent completion and refactoring which are hard to reproduce with tag files and stuff. There are projects like eclim (http://eclim.org/) which provide these features for Vim, Emacs and others, but it requires running the whole Eclipse stack in the background.
A better solution would be to create a separate library or program to provide the same features to text editors without running all the unnecessary Eclipse baggage. It does not even have to be created from scratch, because one could simply figure out how to cut out the necessary parts of the Eclipse Java parser/refactorer and create a wrapper to run it without Eclipse. I'm sure lots of Vim/Emacs/Textmate/etc users would donate money to have such a lightweight support library, so they can use their favorite editor for Java programming too without having to miss intelligent lanugage support features or having to run some behemoth IDE in the background. These donations could enable an open source developer to work on this common library (to be used by all editors) for a while whithout having to take up other jobs. Joey Hess' git annex (http://www.kickstarter.com/projects/joeyh/git-annex-assistant-like-dropbox-but-with-your-own) is a good example for this user funded open source development.
Would you donate money to a project like this? I would. Voice your opinion in the comments, so that people who have the skills to create such an editor-independent completion/refactoring helper for the popular text editors see there is interest and maybe get motivated to start a Kickstarter project for this.
It's a little more ambitious goal than what you're describing, but it does benefit from the fact that it's already being funded (Google is paying Steve's salary).
As for paying for what you've described, I would have to have a very good assurance that something would come of my donation. My initial reaction is "sure, I'd love better tools behind emacs," followed closely by "if it's an easy or interesting problem though, why haven't I already seen it?"
[1] http://bsumm.net/2012/08/11/steve-yegge-and-grok.html