I've done this before with a few jobs, one of which I went back to after I left and the other thing I was pursuing didn't pan out. In the time I was gone I learned a tremendous amount about myself and what's possible. If you can something like this set this up, it's only a win-win.
A few reasons to not try something like this: (A) - you're worried you'll set back any career path you were on track for at google (B) - you're worried they won't take you back
Have no comment on (A) b/c for me I'm more interested in personal growth than specific career track at a specific company.
If (B) holds, why wouldn't they take you back? Top engineers are hard to come by. If you can't make a convincing case that you're in that top category of engineer, then what has working for Google earned you these past few years? (Other than $..)