I don't think the ATO's issued a Class Ruling on this one, so no help on the cost base.
Re "what would need to happen to be able to write off my SP1 shares as a tax loss" - you'd need to transfer beneficial ownership of them. As SP1 is currently just delisted and not in admin or liquidation, you can do that by transferring your shares to someone else. You could also do it by declaring a trust over them.
SP1 Price at posting:
$1.07 Sentiment: Sell Disclosure: Held