1.5 Alpha kind of halfway does this: https://www.voidtools.com/forum/viewtopic.php?t=9793

Halfway because it is fast, but it's fast because it keeps the index entirely within RAM and thus you can't yet throw an arbitrarily-large disk of stuff at it to content-index.

If you select Search > Advanced from the menu, you get a window where you can enter the content to search for. This is available in the normal as well as the alpha version.