I've come across several tools that search users by keyword, however, none that's open source.
I'm not familiar with the MySpace API and at first glance it doesn't seem like this option is available. I was hoping a PHP class exists for this sort of thing. So my question: how do I identify myspace users that have a keyword? Can it done through the API or another route is needed?
I haven't looked at the API, but I would say if you can't find a search function in the API, your best bet would be to write a scraper.
Use PHP (you may need curl as well) to load the MySpace search page with your query. Then parse the source code of that page to find what you want.
If you can't find a keyword lookup in the API, and you can't do a keyword lookup/search through the website, you're unfortunately likely out of luck.