Here fluffy mentions the possibility of maybe modifying a viewkey in such a way that you can also see outgoing transactions. Does anyone have any more info on what exactly he meant or how it would work?
Recently there was a post on Reddit asking almost the same question which generated quite some additional discussion (although without much positive outlook). Here's a comment by me giving an answer to the original question:
I asked moneromooo about this on IRC, and he said that what's mentioned by u/fluffyponyza in the presentation probably refers to an idea that was discussed on IRC a very long time ago and implemented in moneromooo's viewkey-R branch. The basic idea seems to be to generate the tx pubkey somehow deterministically with some nonce such that a view-only wallet (i.e. an auditor) can recognize outgoing transfers. This approach is however quite problematic and unreliable as the wallet owner needs to follow a specific procedure when creating a tx so that the auditor can see outgoing transfers; i.e. there needs to be trust between the two parties for the scheme to work. As such, the development was abandoned.
Here's the log of IRC on 2017-April-10: https://paste.fedoraproject.org/paste/hnmDKpT5U~kJyCHi0NP4BV5M1UNdIGYhyRLivL9gydE=