I doubt the V8 Engine could be easily done so, you'd have to remove the JIT and all of that in order to do so otherwise it will be dependent on having some kind of emulated processor to do it. Though such a thing does exist...
Well for one, this SQLite.js is completely useless without persistence. All you need is some way to get the contents of the database and save it to a file.
That way if you have some new type of architecture that's not supported by SQLite, but you do have a JavaScript VM available you could really easily whip up an SQLite based app without going to the trouble of porting SQLite itself.
It would be interesting to see the performance of Emscripten SQLite.
Can node.js and the V8 engine itself be compiled to JS? Could be interesting for sandboxing.