Would the QtWebkit API be suitable for use ? http://doc.qt.nokia.com/4.7-snapshot/qtwebkit.html Is the QWebElement class and API what you need and mean ?
Are you looking to manipulate the HTML element markup via DOM ?
Are you looking to manipulate the browser context from JavaScript execution from inside the page ?
Is it straightforward enough in Eclipse to start a worker/helper process which can share windows handles to provide a seamless IDE (a bit like how Chrome uses processes per tab and Firefox now puts Flash in a worker process) ?
I believe the Qt suite is able to run across all the desired platforms, Linux, Windows and MaxOSX all in 32bit and 64bit forms. Hopefully I'll have some Maven releases out to allow easier consumption and some Eclipse plugins to follow to build work on top of.