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Let's have Spotify locally save all mobile offline play history and write it to Last.fm scrobble list when you turn it back online. Even better - a Spotify play history on desktop and mobile applications that tracks all play history online or offline.
Updated on 2019-07-11
Sorry for the delay, looks like this one slipped through!
There was an update to Last FM last year which introduced offline scrobbling. The update also means scrobbling works on the Web Player and over devices on Connect. As mentioned by @hans-jürgen, you can check out the article from Last FM here.