it should be extremely easy to have a little (autohotkey for example) script that when you press hotkey copies the selected text and opens up a specific search in an existing or newly launched browser window.
some extras suggested by your idea would be:
1) dont open in normal browser, use a special small popup browser.
2) present user with menu of sites to search.
3) extra stuff.
i like it, good idea. lots of room for extra features.