I won a good deal for a Shimano 2012 SLX FC-M660 set, excluding bottom bracket and crank bolt.
Is there any chance I can upgrade my touring bikes FC-M431 Octalink somehow without changing the chain rings?
If you would like to change to the FC-M660 cranks you are going to have to purchase a new hollowtech ii compatible bottom bracket and change everything. The octalink is a cartridge bottom bracket and the hollowtech ii is an external bearing bottom bracket. The cranks aren't compatible between the two interfaces.
You can get away without a crank bolt as this only provides pre-tension when securing the left crank arm. Borrow one off a friend when fitting your crank then stick it back on the their bike. It makes no difference once the crank is secured.
If your reason for keeping the chain rings is to maintain a gear ratio you can change out the rings, they both use a 4 bolt 104 BCD so the rings are inter-changeable.