Is there an easy way to modify Twitter Bootstrap so that all of its responsive columns work based off of the device-width as opposed to responding to the dimensions of the browser window?

If you did that it would no longer be responsive! (actually, the only way it would 'respond' would be if you're using a tablet/phone and rotated it)

True, but there's still a need for it IMHO.

I want the desktop version to still be responsive between medium/large sizes. But I don't want someone using a desktop to be able to shrink the window to tablet size and lose all the ads. (Since our mobile versions are ad-lite.)